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..1c4afcbfd5a3ac3533470a1cfb348452c052a2a3 100644
--- a/cc/output/swap_promise.h
+++ b/cc/output/swap_promise.h
@@ -39,6 +39,8 @@ 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*().
+//
+// For each SwapPromise, OnCommitForSwapPromises traces its original LatencyInfo

[Yufeng Shen (Slow to review), 2015/06/16 17:32:18]

SwapPromise is generic, so you should only mention here when is OnCommit()
getting called, but move the "trace its original LatencyInfo" into
LatencyInfoSwapPromise::OnCommit() 's implementation.

 class CC_EXPORT SwapPromise {
  public:
   enum DidNotSwapReason {
@@ -54,6 +56,7 @@ class CC_EXPORT SwapPromise {
   virtual void DidActivate() = 0;
   virtual void DidSwap(CompositorFrameMetadata* metadata) = 0;
   virtual void DidNotSwap(DidNotSwapReason reason) = 0;
+  virtual void OnCommitForSwapPromises() {}
 

[Yufeng Shen (Slow to review), 2015/06/16 17:32:18]

ah, I actually meant the naming to be LayerTreeHost::OnCommitForSwapPromises()
and SwapPromise::OnCommit()

   // 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