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

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 7bb993226520f0504b1a44766988e6eeb18cd322..ecfa7d4bb21f8d57d09212f5a46c36f3be76f092 100644
--- a/cc/ipc/mojo_compositor_frame_sink.mojom
+++ b/cc/ipc/mojo_compositor_frame_sink.mojom
@@ -14,6 +14,12 @@ import "cc/ipc/returned_resource.mojom";
 import "cc/ipc/surface_sequence.mojom";
 import "ui/gfx/geometry/mojo/geometry.mojom";
 
+// Allows inspecting activity on a frame sink. This is typically used by an
+// embedder to inspect frame-sink activity of the embedded client.
+interface FrameSinkObserver {
+  OnBeginFrame();
+};
+
 // A MojoCompositorFrameSink is an interface for receiving CompositorFrame
 // structs. A CompositorFrame contains the complete output meant for display.
 // Each time a client has a graphical update, and receives an OnBeginFrame, it
@@ -73,4 +79,10 @@ interface MojoCompositorFrameSinkPrivate {
   // CompositorFrameSink submits to. The result can be in form of a bitmap
   // or a texture. See cc::CopyOutputRequest.
   RequestCopyOfSurface(cc.mojom.CopyOutputRequest request);
+
+  // Requests begin frame messages, and installs a FrameSinkObserver, which is
+  // notified when the frame-sink receives a BeginFrame. This is typically used
+  // by an embedder to inspect frame-sink activity of the embedded client. The
+  // observer keeps receiving begin-frame until it is destroyed.
+  RequestBeginFrames(FrameSinkObserver observer);
 };