Aura-Mus: Enable Surface Synchronization behind flag

ui/aura/mus/client_surface_embedder.cc

Index: ui/aura/mus/client_surface_embedder.cc
diff --git a/ui/aura/mus/client_surface_embedder.cc b/ui/aura/mus/client_surface_embedder.cc
index eab0146fc704b9aed06ceec240ac1b939bbcb1c6..f3610ba94d3c762ed5bdf29ec8c09280a0ffda15 100644
--- a/ui/aura/mus/client_surface_embedder.cc
+++ b/ui/aura/mus/client_surface_embedder.cc
@@ -72,4 +72,10 @@ void ClientSurfaceEmbedder::SetPrimarySurfaceInfo(
       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