ui: fix unnecessary draw cycles on cpu frequency tracks

Ensure that we don't unnecessarily draw data which is out of the window
frame. This significantly reduces the time taken to draw these tracks.

Change-Id: Ib81721a94f9da390222cde14a1e888f2c7560cca

ui/src/tracks/cpu_freq/index.ts

diff --git a/ui/src/tracks/cpu_freq/index.ts b/ui/src/tracks/cpu_freq/index.ts
index f07f263..aa421f6 100644
--- a/ui/src/tracks/cpu_freq/index.ts
+++ b/ui/src/tracks/cpu_freq/index.ts
@@ -383,7 +383,7 @@
     // Draw CPU idle rectangles that overlay the CPU freq graph.
     ctx.fillStyle = `rgba(240, 240, 240, 1)`;
 
-    for (let i = 0; i < data.lastIdleValues.length; i++) {
+    for (let i = startIdx; i < endIdx; i++) {
       if (data.lastIdleValues[i] < 0) {
         continue;
       }