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

ui/aura/mus/window_tree_client.cc

Index: ui/aura/mus/window_tree_client.cc
diff --git a/ui/aura/mus/window_tree_client.cc b/ui/aura/mus/window_tree_client.cc
index 5244c70da3aab0a8c2835aa8be5d51270b5e3e41..2b7e468378eb61ba5d9834efd5be37ea6c2bf57f 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -898,6 +898,12 @@ void WindowTreeClient::OnWindowMusPropertyChanged(
                            transport_value_mojo);
 }
 
+void WindowTreeClient::RequestBeginFrames(
+    WindowMus* window,
+    cc::mojom::FrameSinkObserverPtr observer) {
+  tree_->RequestBeginFrames(window->server_id(), std::move(observer));
+}
+
 void WindowTreeClient::OnWmMoveLoopCompleted(uint32_t change_id,
                                              bool completed) {
   if (window_manager_client_)