Trace main thread commit in flow events.

cc/output/swap_promise.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_OUTPUT_SWAP_PROMISE_H_
 #define CC_OUTPUT_SWAP_PROMISE_H_
 
 #include "cc/output/compositor_frame_metadata.h"
 
 namespace cc {
@@ skipping 36 matching lines @@
     COMMIT_NO_UPDATE,
     ACTIVATION_FAILS,
   };
 
   SwapPromise() {}
   virtual ~SwapPromise() {}
 
   virtual void DidActivate() = 0;
   virtual void DidSwap(CompositorFrameMetadata* metadata) = 0;
   virtual void DidNotSwap(DidNotSwapReason reason) = 0;
+  // This is called when the main thread starts a (blocking) commit
+  virtual void 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
   // visualize the object's path through the system.
   virtual int64 TraceId() const = 0;
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_SWAP_PROMISE_H_