Aura-Mus: Enable Surface Synchronization behind flag

ui/aura/mus/client_surface_embedder.h

Index: ui/aura/mus/client_surface_embedder.h
diff --git a/ui/aura/mus/client_surface_embedder.h b/ui/aura/mus/client_surface_embedder.h
index 6fe1f244ae702bc4276087a26eda6a5edfd21e7f..58d0bf911920e4300ad3cb7c27e60aa622393674 100644
--- a/ui/aura/mus/client_surface_embedder.h
+++ b/ui/aura/mus/client_surface_embedder.h
@@ -30,6 +30,10 @@ class ClientSurfaceEmbedder {
   // 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_;