[cc] Track observer status in ExternalBeginFrameSource.

services/ui/public/cpp/window_compositor_frame_sink.cc

Index: services/ui/public/cpp/window_compositor_frame_sink.cc
diff --git a/services/ui/public/cpp/window_compositor_frame_sink.cc b/services/ui/public/cpp/window_compositor_frame_sink.cc
index 9529895d5c66c955f4647055c28a6e0d868e10d6..84a1419f7b24671cc4d6c68bcd66431b1a34ed5c 100644
--- a/services/ui/public/cpp/window_compositor_frame_sink.cc
+++ b/services/ui/public/cpp/window_compositor_frame_sink.cc
@@ -126,6 +126,10 @@ 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(