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

components/viz/frame_sinks/gpu_root_compositor_frame_sink.cc

Index: components/viz/frame_sinks/gpu_root_compositor_frame_sink.cc
diff --git a/components/viz/frame_sinks/gpu_root_compositor_frame_sink.cc b/components/viz/frame_sinks/gpu_root_compositor_frame_sink.cc
index b614fe8811b7dff661545abf62c54a390754023e..e69d631deac5d7dc50b8214f2f82aaec5047fd00 100644
--- a/components/viz/frame_sinks/gpu_root_compositor_frame_sink.cc
+++ b/components/viz/frame_sinks/gpu_root_compositor_frame_sink.cc
@@ -104,6 +104,13 @@ void GpuRootCompositorFrameSink::RequestCopyOfSurface(
   support_->RequestCopyOfSurface(std::move(request));
 }
 
+void GpuRootCompositorFrameSink::RequestBeginFrames(
+    cc::mojom::FrameSinkObserverPtr observer) {
+  // RequestBeginFrames() should only be usable by embedders. Nothing embeds the
+  // root, so this should never be reached.
+  NOTREACHED();
+}
+
 void GpuRootCompositorFrameSink::DisplayOutputSurfaceLost() {
   // TODO(staraz): Implement this. Client should hear about context/output
   // surface lost.