Aura-Mus: Enable Surface Synchronization behind flag

ui/aura/mus/client_surface_embedder.h

 // 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 <memory>
 
 #include "base/macros.h"
 
 namespace cc {
 class SurfaceInfo;
@@ skipping 12 matching lines @@
 // layer, and updating them when the client submits new surfaces.
 class ClientSurfaceEmbedder {
  public:
   explicit ClientSurfaceEmbedder(Window* window);
   ~ClientSurfaceEmbedder();
 
   // Updates the clip layer and primary SurfaceInfo of the surface layer based
   // on the provided |surface_info|.
   void SetPrimarySurfaceInfo(const cc::SurfaceInfo& surface_info);
 
+  // Sets the fallback SurfaceInfo of the surface layer. The clip layer is not
+  // updated.
+  void SetFallbackSurfaceInfo(const cc::SurfaceInfo& surface_info);
+
  private:
   // The window which embeds the client.
   Window* window_;
 
   // Contains the client's content.
   std::unique_ptr<ui::Layer> surface_layer_;
 
   // Used for clipping the surface layer to the window bounds.
   std::unique_ptr<ui::Layer> clip_layer_;
 
   DISALLOW_COPY_AND_ASSIGN(ClientSurfaceEmbedder);
 };
 
 }  // namespace aura