[ui] Remove trackRegistry & move SliceRect to public.

This patch tidies up track.ts a little.

`TrackRegistry` wasn't being used anymore and SliceRect is used in the
`Track` interface which is defined in public and which plugins make use
of, so it makes sense to move it into public as well.

Change-Id: I6a19d60e4867822a99e1731a624cc93af2bc18f8

ui/src/common/basic_async_track.ts

diff --git a/ui/src/common/basic_async_track.ts b/ui/src/common/basic_async_track.ts
index 41dfe3d..d5e0dda 100644
--- a/ui/src/common/basic_async_track.ts
+++ b/ui/src/common/basic_async_track.ts
@@ -18,8 +18,7 @@
 import {raf} from '../core/raf_scheduler';
 import {globals} from '../frontend/globals';
 import {PxSpan, TimeScale} from '../frontend/time_scale';
-import {SliceRect} from '../frontend/track';
-import {Track, TrackContext} from '../public';
+import {SliceRect, Track, TrackContext} from '../public';
 
 import {TrackData} from './track_data';
 

ui/src/common/track_adapter.ts

diff --git a/ui/src/common/track_adapter.ts b/ui/src/common/track_adapter.ts
index 8dbb089..7514c5f 100644
--- a/ui/src/common/track_adapter.ts
+++ b/ui/src/common/track_adapter.ts
@@ -18,7 +18,8 @@
 import {assertExists} from '../base/logging';
 import {duration, Span, time} from '../base/time';
 import {PxSpan, TimeScale} from '../frontend/time_scale';
-import {NewTrackArgs, SliceRect} from '../frontend/track';
+import {NewTrackArgs} from '../frontend/track';
+import {SliceRect} from '../public';
 import {EngineProxy} from '../trace_processor/engine';
 
 import {BasicAsyncTrack} from './basic_async_track';