[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 2e7876fce090bab15137e51049c32111bb94d3b7..ccfb8cacb92a73c8b6929e3119f4a4771637abd9 100644
--- a/services/ui/public/cpp/window_compositor_frame_sink.cc
+++ b/services/ui/public/cpp/window_compositor_frame_sink.cc
@@ -124,6 +124,10 @@ 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(