[cc] Pass on BeginFrameAcks from CompositorEBFS through RWHVAura, DFH.

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 101 matching lines @@
   // BeginFrameObservers use DidFinishFrame to acknowledge that they have
   // completed handling a BeginFrame.
   //
   // The DisplayScheduler uses these acknowledgments to trigger an early
   // deadline once all BeginFrameObservers have completed a frame.
   //
   // They also provide back pressure to a frame source about frame processing
   // (rather than toggling SetNeedsBeginFrames every frame). For example, the
   // BackToBackFrameSource uses them to make sure only one frame is pending at a
   // time.
+  //
+  // Note that the BeginFrameSource should not assume that the |ack| references
+  // a valid BeginFrame sent by the source. The |ack| may reference a BeginFrame
+  // sent by a different BeginFrameSource, and a malicious client may reference
+  // any invalid frame. The source is responsible for checking for
+  // validity/relevance of the BeginFrame itself.
   // TODO(eseckler): Use BeginFrameAcks in DisplayScheduler as described above.
   virtual void DidFinishFrame(BeginFrameObserver* obs,
                               const BeginFrameAck& ack) = 0;
 
   // Add/Remove an observer from the source. When no observers are added the BFS
   // should shut down its timers, disable vsync, etc.
   virtual void AddObserver(BeginFrameObserver* obs) = 0;
   virtual void RemoveObserver(BeginFrameObserver* obs) = 0;
 
   // Returns false if the begin frame source will just continue to produce
@@ skipping 180 matching lines @@
   bool frame_active_ = false;
   BeginFrameObserverAckTracker ack_tracker_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ExternalBeginFrameSource);
 };
 
 }  // namespace cc
 
 #endif  // CC_SCHEDULER_BEGIN_FRAME_SOURCE_H_