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

ui/aura/mus/window_port_mus.h

Index: ui/aura/mus/window_port_mus.h
diff --git a/ui/aura/mus/window_port_mus.h b/ui/aura/mus/window_port_mus.h
index b7a933bf1d1cf27e7d975b1e6eb4d83e899d9f0f..e024fd5a2a37b5dde33f0c4afdf5957c3e9964ed 100644
--- a/ui/aura/mus/window_port_mus.h
+++ b/ui/aura/mus/window_port_mus.h
@@ -93,6 +93,8 @@ class AURA_EXPORT WindowPortMus : public WindowPort, public WindowMus {
           compositor_frame_sink_binding);
 
  private:
+  class FrameSinkObserver;
+
   friend class WindowPortMusTestApi;
   friend class WindowTreeClient;
   friend class WindowTreeClientPrivate;
@@ -264,6 +266,8 @@ class AURA_EXPORT WindowPortMus : public WindowPort, public WindowMus {
                          std::unique_ptr<ui::PropertyData> data) override;
   std::unique_ptr<cc::CompositorFrameSink> CreateCompositorFrameSink() override;
   cc::SurfaceId GetSurfaceId() const override;
+  void SetNeedsBeginFrames(bool needs,
+                           base::RepeatingClosure callback) override;
   void OnWindowAddedToRootWindow() override {}
   void OnWillRemoveWindowFromRootWindow() override {}
 
@@ -292,6 +296,8 @@ class AURA_EXPORT WindowPortMus : public WindowPort, public WindowMus {
 
   ui::CursorData cursor_;
 
+  std::unique_ptr<FrameSinkObserver> frame_sink_observer_;
+
   DISALLOW_COPY_AND_ASSIGN(WindowPortMus);
 };