Aura-Mus: Implement gutter in the embedder.

ui/compositor/layer.h

Index: ui/compositor/layer.h
diff --git a/ui/compositor/layer.h b/ui/compositor/layer.h
index bbb0962ef3416242b09fde04b4143a91a0dea430..a97525d03f1064451efea4386ec45f024d7dd408 100644
--- a/ui/compositor/layer.h
+++ b/ui/compositor/layer.h
@@ -303,6 +303,9 @@ class COMPOSITOR_EXPORT Layer
   // display compositor, the fallback surface will be used.
   void SetFallbackSurface(const cc::SurfaceInfo& surface_info);
 
+  // Returns the fallback SurfaceInfo set by SetFallbackSurface.
+  const cc::SurfaceInfo* GetFallbackSurfaceInfo() const;
+
   bool has_external_content() {
     return texture_layer_.get() || surface_layer_.get();
   }