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

services/ui/ws/server_window_compositor_frame_sink_manager.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "services/ui/ws/server_window_compositor_frame_sink_manager.h"
 
 #include <utility>
 
 #include "mojo/public/cpp/bindings/strong_binding.h"
 #include "services/ui/ws/ids.h"
@@ skipping 45 matching lines @@
 
 void ServerWindowCompositorFrameSinkManager::ClaimTemporaryReference(
     const cc::SurfaceId& surface_id) {
   if (!compositor_frame_sink_.is_bound()) {
     pending_compositor_frame_sink_request_ =
         mojo::MakeRequest(&compositor_frame_sink_);
   }
   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