[ui] Rename `trackId` (i.e. UI track IDs) to `trackKey`, or just `key`
where appropriate.

Change-Id: I7c994b6328fdd22334c3369630e996e52149493b
diff --git a/ui/src/frontend/track_panel.ts b/ui/src/frontend/track_panel.ts
index 5e39589..90a593a 100644
--- a/ui/src/frontend/track_panel.ts
+++ b/ui/src/frontend/track_panel.ts
@@ -106,8 +106,8 @@
     let highlightClass = '';
     const searchIndex = globals.state.searchIndex;
     if (searchIndex !== -1) {
-      const trackId = globals.currentSearchResults.trackIds[searchIndex];
-      if (trackId === attrs.trackState.id) {
+      const trackKey = globals.currentSearchResults.trackKeys[searchIndex];
+      if (trackKey === attrs.trackState.key) {
         highlightClass = 'flash';
       }
     }
@@ -140,12 +140,12 @@
           m(TrackButton, {
             action: () => {
               globals.dispatch(
-                  Actions.toggleTrackPinned({trackId: attrs.trackState.id}));
+                  Actions.toggleTrackPinned({trackKey: attrs.trackState.key}));
             },
             i: Icons.Pin,
-            filledIcon: isPinned(attrs.trackState.id),
-            tooltip: isPinned(attrs.trackState.id) ? 'Unpin' : 'Pin to top',
-            showButton: isPinned(attrs.trackState.id),
+            filledIcon: isPinned(attrs.trackState.key),
+            tooltip: isPinned(attrs.trackState.key) ? 'Unpin' : 'Pin to top',
+            showButton: isPinned(attrs.trackState.key),
             fullHeight: true,
           }),
           globals.state.currentSelection !== null &&
@@ -153,12 +153,12 @@
               m(TrackButton, {
                 action: (e: MouseEvent) => {
                   globals.dispatch(Actions.toggleTrackSelection(
-                      {id: attrs.trackState.id, isTrackGroup: false}));
+                      {id: attrs.trackState.key, isTrackGroup: false}));
                   e.stopPropagation();
                 },
-                i: isSelected(attrs.trackState.id) ? Icons.Checkbox :
-                                                     Icons.BlankCheckbox,
-                tooltip: isSelected(attrs.trackState.id) ?
+                i: isSelected(attrs.trackState.key) ? Icons.Checkbox :
+                                                      Icons.BlankCheckbox,
+                tooltip: isSelected(attrs.trackState.key) ?
                     'Remove track' :
                     'Add track to selection',
                 showButton: true,
@@ -171,7 +171,7 @@
     if (dataTransfer === null) return;
     this.dragging = true;
     raf.scheduleFullRedraw();
-    dataTransfer.setData('perfetto/track', `${this.attrs!.trackState.id}`);
+    dataTransfer.setData('perfetto/track', `${this.attrs!.trackState.key}`);
     dataTransfer.setDragImage(new Image(), 0, 0);
   }
 
@@ -210,7 +210,7 @@
     if (dataTransfer === null) return;
     raf.scheduleFullRedraw();
     const srcId = dataTransfer.getData('perfetto/track');
-    const dstId = this.attrs!.trackState.id;
+    const dstId = this.attrs!.trackState.key;
     globals.dispatch(Actions.moveTrack({srcId, op: this.dropping, dstId}));
     this.dropping = undefined;
   }
@@ -289,7 +289,7 @@
           style: {
             height: `${Math.max(18, attrs.track.getHeight())}px`,
           },
-          id: 'track_' + attrs.trackState.id,
+          id: 'track_' + attrs.trackState.key,
         },
         [
           m(TrackShell, {track: attrs.track, trackState: attrs.trackState}),
@@ -298,9 +298,9 @@
   }
 
   oncreate({attrs}: m.CVnode<TrackComponentAttrs>) {
-    if (globals.frontendLocalState.scrollToTrackId === attrs.trackState.id) {
-      verticalScrollToTrack(attrs.trackState.id);
-      globals.frontendLocalState.scrollToTrackId = undefined;
+    if (globals.frontendLocalState.scrollToTrackKey === attrs.trackState.key) {
+      verticalScrollToTrack(attrs.trackState.key);
+      globals.frontendLocalState.scrollToTrackKey = undefined;
     }
   }
 }
@@ -332,7 +332,7 @@
 }
 
 interface TrackPanelAttrs {
-  id: string;
+  trackKey: string;
   selectable: boolean;
 }
 
@@ -344,22 +344,22 @@
   private trackState: TrackState|undefined;
 
   private tryLoadTrack(vnode: m.CVnode<TrackPanelAttrs>) {
-    const trackId = vnode.attrs.id;
-    const trackState = globals.state.tracks[trackId];
+    const trackKey = vnode.attrs.trackKey;
+    const trackState = globals.state.tracks[trackKey];
 
     if (!trackState) return;
 
-    const {id, uri, params} = trackState;
+    const {uri, params} = trackState;
 
     const trackCtx: TrackContext = {
-      trackInstanceId: id,
+      trackKey,
       mountStore: <T>(migrate: Migrate<T>) => {
         const {store, state} = globals;
-        const migratedState = migrate(state.tracks[trackId].state);
+        const migratedState = migrate(state.tracks[trackKey].state);
         globals.store.edit((draft) => {
-          draft.tracks[trackId].state = migratedState;
+          draft.tracks[trackKey].state = migratedState;
         });
-        return store.createProxy<T>(['tracks', trackId, 'state']);
+        return store.createProxy<T>(['tracks', trackKey, 'state']);
       },
       params,
     };
@@ -409,7 +409,7 @@
     }
     const selectedArea = globals.state.areas[selection.areaId];
     const selectedAreaDuration = selectedArea.end - selectedArea.start;
-    if (selectedArea.tracks.includes(trackState.id)) {
+    if (selectedArea.tracks.includes(trackState.key)) {
       ctx.fillStyle = SELECTION_FILL_COLOR;
       ctx.fillRect(
           visibleTimeScale.timeToPx(selectedArea.start) + TRACK_SHELL_WIDTH,