Reimplement vsync latency UMA to be based on BeginImplFrame rather than BeginMainFrame

cc/scheduler/scheduler.h

 // Copyright 2011 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_SCHEDULER_SCHEDULER_H_
 #define CC_SCHEDULER_SCHEDULER_H_
 
 #include <deque>
 #include <memory>
 #include <string>
@@ skipping 188 matching lines @@
   bool ShouldRecoverImplLatency(const BeginFrameArgs& args,
                                 bool can_activate_before_deadline) const;
   bool CanBeginMainFrameAndActivateBeforeDeadline(
       const BeginFrameArgs& args,
       base::TimeDelta bmf_to_activate_estimate,
       base::TimeTicks now) const;
   void AdvanceCommitStateIfPossible();
   bool IsBeginMainFrameSentOrStarted() const;
   void BeginImplFrameWithDeadline(const BeginFrameArgs& args);
   void BeginImplFrameSynchronous(const BeginFrameArgs& args);
-  void BeginImplFrame(const BeginFrameArgs& args);
+  void BeginImplFrame(const BeginFrameArgs& args, base::TimeTicks now);
   void FinishImplFrame();
   enum BeginFrameResult { kBeginFrameSkipped, kBeginFrameFinished };
   void SendBeginFrameAck(const BeginFrameArgs& args, BeginFrameResult result);
   void OnBeginImplFrameDeadline();
   void PollToAdvanceCommitState();
 
   bool IsInsideAction(SchedulerStateMachine::Action action) {
     return inside_action_ == action;
   }
 
   base::WeakPtrFactory<Scheduler> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(Scheduler);
 };
 
 }  // namespace cc
 
 #endif  // CC_SCHEDULER_SCHEDULER_H_