cc: Introduce BeginFrame sequence numbers and acknowledgements.

ui/aura/mus/window_compositor_frame_sink.cc

Index: ui/aura/mus/window_compositor_frame_sink.cc
diff --git a/ui/aura/mus/window_compositor_frame_sink.cc b/ui/aura/mus/window_compositor_frame_sink.cc
index 7d252b89d69e1edbe206a486c1f95342d9988f0a..7054ef74a178ff15b5a547242095b305b5c91556 100644
--- a/ui/aura/mus/window_compositor_frame_sink.cc
+++ b/ui/aura/mus/window_compositor_frame_sink.cc
@@ -5,6 +5,7 @@
 #include "ui/aura/mus/window_compositor_frame_sink.h"
 
 #include "base/bind.h"
+#include "cc/output/begin_frame_args.h"
 #include "cc/output/compositor_frame_sink_client.h"
 #include "gpu/ipc/client/gpu_channel_host.h"
 
@@ -119,6 +120,12 @@ void WindowCompositorFrameSink::OnNeedsBeginFrames(bool needs_begin_frames) {
   compositor_frame_sink_->SetNeedsBeginFrame(needs_begin_frames);
 }
 
+void WindowCompositorFrameSink::OnDidFinishFrame(const cc::BeginFrameAck& ack) {
+  // If there were updates, SubmitCompositorFrame serves as ack.
+  if (!ack.has_updates)
+    compositor_frame_sink_->BeginFrameDidNotDraw(ack);
+}
+
 WindowCompositorFrameSinkBinding::~WindowCompositorFrameSinkBinding() {}
 
 WindowCompositorFrameSinkBinding::WindowCompositorFrameSinkBinding(