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

components/viz/frame_sinks/gpu_compositor_frame_sink.h

Index: components/viz/frame_sinks/gpu_compositor_frame_sink.h
diff --git a/components/viz/frame_sinks/gpu_compositor_frame_sink.h b/components/viz/frame_sinks/gpu_compositor_frame_sink.h
index 1d0e043362b6ce6f07e08b2ef97d323a8e920e64..ace8fd8c12a18c4a4fcfe51154b32cd4370a92c4 100644
--- a/components/viz/frame_sinks/gpu_compositor_frame_sink.h
+++ b/components/viz/frame_sinks/gpu_compositor_frame_sink.h
@@ -10,6 +10,7 @@
 
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
+#include "cc/ipc/frame_sink_manager.mojom.h"
 #include "cc/ipc/mojo_compositor_frame_sink.mojom.h"
 #include "cc/surfaces/compositor_frame_sink_support.h"
 #include "cc/surfaces/compositor_frame_sink_support_client.h"
@@ -47,6 +48,7 @@ class GpuCompositorFrameSink
   void ClaimTemporaryReference(const cc::SurfaceId& surface_id) override;
   void RequestCopyOfSurface(
       std::unique_ptr<cc::CopyOutputRequest> request) override;
+  void RequestBeginFrames(cc::mojom::FrameSinkObserverPtr observer) override;
 
  private:
   // cc::CompositorFrameSinkSupportClient implementation:
@@ -66,6 +68,8 @@ class GpuCompositorFrameSink
   bool client_connection_lost_ = false;
   bool private_connection_lost_ = false;
 
+  cc::mojom::FrameSinkObserverPtr observer_;
+
   cc::mojom::MojoCompositorFrameSinkClientPtr client_;
   mojo::Binding<cc::mojom::MojoCompositorFrameSink>
       compositor_frame_sink_binding_;