[cc] Track observer status in ExternalBeginFrameSource.

cc/surfaces/direct_compositor_frame_sink.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_SURFACES_DIRECT_COMPOSITOR_FRAME_SINK_H_
 #define CC_SURFACES_DIRECT_COMPOSITOR_FRAME_SINK_H_
 
 #include "base/macros.h"
 #include "base/threading/thread_checker.h"
 #include "cc/output/compositor_frame_sink.h"
@@ skipping 51 matching lines @@
  private:
   // CompositorFrameSinkSupportClient implementation:
   void DidReceiveCompositorFrameAck() override;
   void OnBeginFrame(const BeginFrameArgs& args) override;
   void ReclaimResources(const ReturnedResourceArray& resources) override;
   void WillDrawSurface(const LocalSurfaceId& local_surface_id,
                        const gfx::Rect& damage_rect) override;
 
   // ExternalBeginFrameSourceClient implementation:
   void OnNeedsBeginFrames(bool needs_begin_frame) override;
+  void OnDidFinishFrame(const BeginFrameAck& ack) override;
 
   // This class is only meant to be used on a single thread.
   base::ThreadChecker thread_checker_;
 
   const FrameSinkId frame_sink_id_;
   LocalSurfaceId delegated_local_surface_id_;
   SurfaceManager* surface_manager_;
   SurfaceIdAllocator surface_id_allocator_;
   Display* display_;
   gfx::Size last_swap_frame_size_;
   bool is_lost_ = false;
   std::unique_ptr<CompositorFrameSinkSupport> support_;
   std::unique_ptr<ExternalBeginFrameSource> begin_frame_source_;
 
   DISALLOW_COPY_AND_ASSIGN(DirectCompositorFrameSink);
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_DIRECT_COMPOSITOR_FRAME_SINK_H_