cc: Record frame timing composite events in the tracker.

cc/debug/frame_timing_request.h

Index: cc/debug/frame_timing_request.h
diff --git a/cc/debug/frame_timing_request.h b/cc/debug/frame_timing_request.h
index 7ab97def0ab49a4e04991a4957b8beef3075a9f7..aa5b83ad51f7250f575d8186cc25197bf11ff588 100644
--- a/cc/debug/frame_timing_request.h
+++ b/cc/debug/frame_timing_request.h
@@ -18,7 +18,10 @@ class FrameTimingRequest {
   FrameTimingRequest();
   FrameTimingRequest(int64_t request_id, const gfx::Rect& rect);
 
+  // Return the ID for the request.
   int64_t id() const { return id_; }
+
+  // Return the layer space rect for this request.
   const gfx::Rect& rect() const { return rect_; }
 
  private: