Aura-Mus: Implement gutter in the embedder.

ui/compositor/layer.cc

Index: ui/compositor/layer.cc
diff --git a/ui/compositor/layer.cc b/ui/compositor/layer.cc
index bbbc7af34b51b6efc4b7c6c31bb0dfb8dd26e86a..57fd1ccf340ca04d0b163d3707c027bb14575e46 100644
--- a/ui/compositor/layer.cc
+++ b/ui/compositor/layer.cc
@@ -686,6 +686,12 @@ void Layer::SetFallbackSurface(const cc::SurfaceInfo& surface_info) {
     mirror->dest()->SetFallbackSurface(surface_info);
 }
 
+const cc::SurfaceInfo* Layer::GetFallbackSurfaceInfo() const {
+  if (surface_layer_)
+    return &surface_layer_->fallback_surface_info();
+  return nullptr;
+}
+
 void Layer::SetShowSolidColorContent() {
   DCHECK_EQ(type_, LAYER_SOLID_COLOR);