cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/ipc/mojo_compositor_frame_sink.mojom

Index: cc/ipc/mojo_compositor_frame_sink.mojom
diff --git a/cc/ipc/mojo_compositor_frame_sink.mojom b/cc/ipc/mojo_compositor_frame_sink.mojom
index 5c6d77e38e494c907a98115ce8aa65301b3db908..0d1c99c45974b9b18e50e99f47f4d95d94790a11 100644
--- a/cc/ipc/mojo_compositor_frame_sink.mojom
+++ b/cc/ipc/mojo_compositor_frame_sink.mojom
@@ -35,6 +35,10 @@ interface MojoCompositorFrameSink {
   // that its resources gets returned in time.
   EvictFrame();
 
+  // Notifies the frame sink that a BeginFrame was completed, but that no
+  // CompositorFrame was produced as a result of it.
+  BeginFrameDidNotDraw(BeginFrameAck ack);
+
   // TODO(fsamuel): ReadbackBitmap API would be useful here.
 };