perfetto-ui: Narrow CPU tracks

- Reduce height of CPU tracks
- Layout up/down buttons side by side
- Center and add a background to tooltip text to make
  it visible

Change-Id: I1e797cac58f376c92db9e037c354820e74c08770
diff --git a/ui/src/frontend/track.ts b/ui/src/frontend/track.ts
index 447f775..bc97ad8 100644
--- a/ui/src/frontend/track.ts
+++ b/ui/src/frontend/track.ts
@@ -42,7 +42,7 @@
   constructor(protected trackState: TrackState) {}
   abstract renderCanvas(ctx: CanvasRenderingContext2D): void;
   getHeight(): number {
-    return 70;
+    return 40;
   }
   onMouseMove(_position: {x: number, y: number}) {}
   onMouseOut() {}