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

components/viz/frame_sinks/gpu_root_compositor_frame_sink.cc

 // Copyright 2017 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 "components/viz/frame_sinks/gpu_root_compositor_frame_sink.h"
 
 #include "cc/surfaces/compositor_frame_sink_support.h"
 #include "cc/surfaces/display.h"
 
 namespace viz {
@@ skipping 86 matching lines @@
 void GpuRootCompositorFrameSink::ClaimTemporaryReference(
     const cc::SurfaceId& surface_id) {
   support_->ClaimTemporaryReference(surface_id);
 }
 
 void GpuRootCompositorFrameSink::RequestCopyOfSurface(
     std::unique_ptr<cc::CopyOutputRequest> request) {
   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.
 }
 
 void GpuRootCompositorFrameSink::DisplayWillDrawAndSwap(
     bool will_draw_and_swap,
     const cc::RenderPassList& render_pass) {}
 
 void GpuRootCompositorFrameSink::DisplayDidDrawAndSwap() {}
@@ skipping 27 matching lines @@
 }
 
 void GpuRootCompositorFrameSink::OnPrivateConnectionLost() {
   private_connection_lost_ = true;
   // Request destruction of |this| only if both connections are lost.
   delegate_->OnPrivateConnectionLost(support_->frame_sink_id(),
                                      client_connection_lost_);
 }
 
 }  // namespace viz