Trace main thread commit in flow events.

cc/output/swap_promise.h

Index: cc/output/swap_promise.h
diff --git a/cc/output/swap_promise.h b/cc/output/swap_promise.h
index 45deee834018bdca7b1f7e2d104ab725ea54bee3..6b7f06b5ce7c94043e8df7d5f4962e50c2528d21 100644
--- a/cc/output/swap_promise.h
+++ b/cc/output/swap_promise.h
@@ -39,6 +39,7 @@ namespace cc {
 // assume that Did*() methods are called at a particular thread. It is better
 // to let the subclass carry thread-safe member data and operate on that
 // member data in Did*().
+//

[Yufeng Shen (Slow to review), 2015/06/16 17:54:16]

extra line

 class CC_EXPORT SwapPromise {
  public:
   enum DidNotSwapReason {
@@ -54,6 +55,8 @@ class CC_EXPORT SwapPromise {
   virtual void DidActivate() = 0;
   virtual void DidSwap(CompositorFrameMetadata* metadata) = 0;
   virtual void DidNotSwap(DidNotSwapReason reason) = 0;
+  // OnCommit is only called for LatencyInfoSwapPromise
+  virtual void OnCommit() {}

[Yufeng Shen (Slow to review), 2015/06/16 17:54:16]

Should not mention subclass's behavior, only mention under what conditionis this
callback triggered, e,g,

"This is called when main thread starts commit."

 
   // A non-zero trace id identifies a trace flow object that is embedded in the
   // swap promise. This can be used for registering additional flow steps to