[cc] Track observer status in ExternalBeginFrameSource.

services/ui/public/cpp/window_compositor_frame_sink.cc

 // 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.
 
 #include "services/ui/public/cpp/window_compositor_frame_sink.h"
 
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
 #include "cc/output/compositor_frame.h"
 #include "cc/output/compositor_frame_sink_client.h"
@@ skipping 106 matching lines @@
 }
 
 void WindowCompositorFrameSink::WillDrawSurface() {
   // TODO(fsamuel, staraz): Implement this.
 }
 
 void WindowCompositorFrameSink::OnNeedsBeginFrames(bool needs_begin_frames) {
   compositor_frame_sink_->SetNeedsBeginFrame(needs_begin_frames);
 }
 
+void WindowCompositorFrameSink::OnDidFinishFrame(const cc::BeginFrameAck& ack) {
+  // TODO(eseckler): Pass on the ack to compositor_frame_sink_.
+}
+
 WindowCompositorFrameSinkBinding::~WindowCompositorFrameSinkBinding() {}
 
 WindowCompositorFrameSinkBinding::WindowCompositorFrameSinkBinding(
     cc::mojom::MojoCompositorFrameSinkRequest compositor_frame_sink_request,
     cc::mojom::MojoCompositorFrameSinkClientPtrInfo
         compositor_frame_sink_client)
     : compositor_frame_sink_request_(std::move(compositor_frame_sink_request)),
       compositor_frame_sink_client_(std::move(compositor_frame_sink_client)) {}
 
 cc::mojom::MojoCompositorFrameSinkRequest
 WindowCompositorFrameSinkBinding::TakeFrameSinkRequest() {
   return std::move(compositor_frame_sink_request_);
 }
 
 cc::mojom::MojoCompositorFrameSinkClientPtrInfo
 WindowCompositorFrameSinkBinding::TakeFrameSinkClient() {
   return std::move(compositor_frame_sink_client_);
 }
 
 }  // namespace ui