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

components/viz/frame_sinks/gpu_compositor_frame_sink.cc

Index: components/viz/frame_sinks/gpu_compositor_frame_sink.cc
diff --git a/components/viz/frame_sinks/gpu_compositor_frame_sink.cc b/components/viz/frame_sinks/gpu_compositor_frame_sink.cc
index 81acb033feac9d9e22793a384d1266586768d906..b8b2f1542a4e75c5ee441f661782b03fd7d569f5 100644
--- a/components/viz/frame_sinks/gpu_compositor_frame_sink.cc
+++ b/components/viz/frame_sinks/gpu_compositor_frame_sink.cc
@@ -71,9 +71,16 @@ void GpuCompositorFrameSink::RequestCopyOfSurface(
   support_->RequestCopyOfSurface(std::move(request));
 }
 
+void GpuCompositorFrameSink::RequestBeginFrame(
+    cc::mojom::FrameSinkObserverPtr observer) {
+  observer_ = std::move(observer);
+}
+
 void GpuCompositorFrameSink::OnBeginFrame(const cc::BeginFrameArgs& args) {
   if (client_)
     client_->OnBeginFrame(args);
+  if (observer_.is_bound())
+    observer_->OnBeginFrame();
 }
 
 void GpuCompositorFrameSink::ReclaimResources(