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

ui/aura/window_port.h

Index: ui/aura/window_port.h
diff --git a/ui/aura/window_port.h b/ui/aura/window_port.h
index 6774dc6edb47012e42e7813563be2f1c824e2c20..c3454ff298399d1d6f3d8bd52c115286465d68f2 100644
--- a/ui/aura/window_port.h
+++ b/ui/aura/window_port.h
@@ -83,6 +83,11 @@ class AURA_EXPORT WindowPort {
   // Get the current cc::SurfaceId.
   virtual cc::SurfaceId GetSurfaceId() const = 0;
 
+  // Requests to be notified (via |callback|) when the client app embedded
+  // inside this window receives a BeginFrame.
+  virtual void SetNeedsBeginFrames(bool needs,
+                                   base::RepeatingClosure callback) = 0;
+
   virtual void OnWindowAddedToRootWindow() = 0;
   virtual void OnWillRemoveWindowFromRootWindow() = 0;