Aura-Mus: Enable Surface Synchronization behind flag

ui/aura/mus/client_surface_embedder.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 "ui/aura/mus/client_surface_embedder.h"
 
 #include "cc/surfaces/surface_reference_factory.h"
 #include "ui/aura/window.h"
 
 namespace aura {
@@ skipping 54 matching lines @@
   surface_layer_->SetBounds(
       gfx::Rect(0, 0, frame_size.width(), frame_size.height()));
   // Clip to window bounds.
   clip_layer_->SetBounds(
       gfx::Rect(0, 0, window_->bounds().width(), window_->bounds().height()));
 
   surface_layer_->SetShowPrimarySurface(
       surface_info, make_scoped_refptr(new StubSurfaceReferenceFactory));
 }
 
+void ClientSurfaceEmbedder::SetFallbackSurfaceInfo(
+    const cc::SurfaceInfo& surface_info) {
+  surface_layer_->SetFallbackSurface(
+      surface_info, make_scoped_refptr(new StubSurfaceReferenceFactory));
+}
+
 }  // namespace aura