Aura-Mus: Enable Surface Synchronization behind flag

ui/compositor/layer.cc

Index: ui/compositor/layer.cc
diff --git a/ui/compositor/layer.cc b/ui/compositor/layer.cc
index c235821bf6ae9a4e778f73442b8a7988169fae50..bbbc7af34b51b6efc4b7c6c31bb0dfb8dd26e86a 100644
--- a/ui/compositor/layer.cc
+++ b/ui/compositor/layer.cc
@@ -675,6 +675,17 @@ void Layer::SetShowPrimarySurface(
     mirror->dest()->SetShowPrimarySurface(surface_info, ref_factory);
 }
 
+void Layer::SetFallbackSurface(const cc::SurfaceInfo& surface_info) {
+  DCHECK(type_ == LAYER_TEXTURED || type_ == LAYER_SOLID_COLOR);
+  DCHECK(surface_layer_);
+
+  // TODO(fsamuel): We should compute the gutter in the display compositor.
+  surface_layer_->SetFallbackSurfaceInfo(surface_info);
+
+  for (const auto& mirror : mirrors_)
+    mirror->dest()->SetFallbackSurface(surface_info);
+}
+
 void Layer::SetShowSolidColorContent() {
   DCHECK_EQ(type_, LAYER_SOLID_COLOR);