Aura-Mus: Enable Surface Synchronization behind flag

ui/aura/mus/window_port_mus.h

 // Copyright 2016 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.
 
 #ifndef UI_AURA_MUS_WINDOW_PORT_MUS_H_
 #define UI_AURA_MUS_WINDOW_PORT_MUS_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 210 matching lines @@
   void SetVisibleFromServer(bool visible) override;
   void SetOpacityFromServer(float opacity) override;
   void SetPredefinedCursorFromServer(ui::mojom::CursorType cursor) override;
   void SetPropertyFromServer(
       const std::string& property_name,
       const std::vector<uint8_t>* property_data) override;
   void SetFrameSinkIdFromServer(const cc::FrameSinkId& frame_sink_id) override;
   const cc::LocalSurfaceId& GetOrAllocateLocalSurfaceId(
       const gfx::Size& surface_size) override;
   void SetPrimarySurfaceInfo(const cc::SurfaceInfo& surface_info) override;
+  void SetFallbackSurfaceInfo(const cc::SurfaceInfo& surface_info) override;
   void DestroyFromServer() override;
   void AddTransientChildFromServer(WindowMus* child) override;
   void RemoveTransientChildFromServer(WindowMus* child) override;
   ChangeSource OnTransientChildAdded(WindowMus* child) override;
   ChangeSource OnTransientChildRemoved(WindowMus* child) override;
   const cc::LocalSurfaceId& GetLocalSurfaceId() override;
   std::unique_ptr<WindowMusChangeData> PrepareForServerBoundsChange(
       const gfx::Rect& bounds) override;
   std::unique_ptr<WindowMusChangeData> PrepareForServerVisibilityChange(
       bool value) override;
@@ skipping 39 matching lines @@
   gfx::Size last_surface_size_;
 
   ui::mojom::CursorType predefined_cursor_ = ui::mojom::CursorType::CURSOR_NULL;
 
   DISALLOW_COPY_AND_ASSIGN(WindowPortMus);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_MUS_WINDOW_PORT_MUS_H_