[cc] Remove remaining_frames from BeginFrameAck.

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 152 matching lines @@
  public:
   ~SyntheticBeginFrameSource() override;
 
   virtual void OnUpdateVSyncParameters(base::TimeTicks timebase,
                                        base::TimeDelta interval) = 0;
   // This overrides any past or future interval from updating vsync parameters.
   virtual void SetAuthoritativeVSyncInterval(base::TimeDelta interval) = 0;
 };
 
 // A frame source which calls BeginFrame (at the next possible time) as soon as
-// remaining frames reaches zero.
+// an observer acknowledges the prior BeginFrame.
 class CC_EXPORT BackToBackBeginFrameSource : public SyntheticBeginFrameSource,
                                              public DelayBasedTimeSourceClient {
  public:
   explicit BackToBackBeginFrameSource(
       std::unique_ptr<DelayBasedTimeSource> time_source);
   ~BackToBackBeginFrameSource() override;
 
   // BeginFrameSource implementation.
   void AddObserver(BeginFrameObserver* obs) override;
   void RemoveObserver(BeginFrameObserver* obs) override;
@@ skipping 134 matching lines @@
   bool frame_active_ = false;
   BeginFrameObserverAckTracker ack_tracker_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ExternalBeginFrameSource);
 };
 
 }  // namespace cc
 
 #endif  // CC_SCHEDULER_BEGIN_FRAME_SOURCE_H_