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

services/ui/ws/server_window_compositor_frame_sink_manager.cc

Index: services/ui/ws/server_window_compositor_frame_sink_manager.cc
diff --git a/services/ui/ws/server_window_compositor_frame_sink_manager.cc b/services/ui/ws/server_window_compositor_frame_sink_manager.cc
index 94ab45b5303db9ef9558576bd214219a9f40dffc..142b7b12ab566c956f23b5d377b5031c5e97a30b 100644
--- a/services/ui/ws/server_window_compositor_frame_sink_manager.cc
+++ b/services/ui/ws/server_window_compositor_frame_sink_manager.cc
@@ -63,5 +63,14 @@ void ServerWindowCompositorFrameSinkManager::ClaimTemporaryReference(
   compositor_frame_sink_->ClaimTemporaryReference(surface_id);
 }
 
+void ServerWindowCompositorFrameSinkManager::RequestBeginFrames(
+    cc::mojom::FrameSinkObserverPtr observer) {
+  if (!compositor_frame_sink_.is_bound()) {
+    pending_compositor_frame_sink_request_ =
+        mojo::MakeRequest(&compositor_frame_sink_);
+  }
+  compositor_frame_sink_->RequestBeginFrames(std::move(observer));
+}
+
 }  // namespace ws
 }  // namespace ui