Fix Omnibox.CharTypedToRepaintLatency regression with D3DVsync experiment

cc/scheduler/begin_frame_source.h

 // Copyright 2014 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_BEGIN_FRAME_SOURCE_H_
 #define CC_SCHEDULER_BEGIN_FRAME_SOURCE_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 257 matching lines @@
   void RemoveObserver(BeginFrameObserver* obs) override;
   void DidFinishFrame(BeginFrameObserver* obs,
                       const BeginFrameAck& ack) override;
   bool IsThrottled() const override;
   void AsValueInto(base::trace_event::TracedValue* state) const override;
 
   void OnSetBeginFrameSourcePaused(bool paused);
   void OnBeginFrame(const BeginFrameArgs& args);
 
  protected:
+  // Called on AddObserver and gets missed BeginFrameArgs for the given
+  // observer.
+  virtual BeginFrameArgs GetMissedBeginFrameArgs(BeginFrameObserver* obs);
+
   BeginFrameArgs last_begin_frame_args_;
   std::unordered_set<BeginFrameObserver*> observers_;
   ExternalBeginFrameSourceClient* client_;
   bool paused_ = false;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ExternalBeginFrameSource);
 };
 
 }  // namespace cc
 
 #endif  // CC_SCHEDULER_BEGIN_FRAME_SOURCE_H_