mus: Embedder can request and observe BeginFrame for embedded client.

ui/aura/local/window_port_local.cc

Index: ui/aura/local/window_port_local.cc
diff --git a/ui/aura/local/window_port_local.cc b/ui/aura/local/window_port_local.cc
index 2ca8fd74ef4ec69fe48713fb93e16172a32d8329..1a9dae63dd68f75417c529480340378bd8eab24e 100644
--- a/ui/aura/local/window_port_local.cc
+++ b/ui/aura/local/window_port_local.cc
@@ -111,6 +111,11 @@ cc::SurfaceId WindowPortLocal::GetSurfaceId() const {
   return cc::SurfaceId(frame_sink_id_, local_surface_id_);
 }
 
+void WindowPortLocal::SetNeedsBeginFrames(bool needs,
+                                          base::RepeatingClosure callback) {
+  NOTREACHED();
+}
+
 void WindowPortLocal::OnWindowAddedToRootWindow() {
   if (frame_sink_id_.is_valid())
     window_->layer()->GetCompositor()->AddFrameSink(frame_sink_id_);