Generalize layer mirroring for phantom windows

ui/compositor/layer_owner.h

Index: ui/compositor/layer_owner.h
diff --git a/ui/compositor/layer_owner.h b/ui/compositor/layer_owner.h
index fd0606c442b4a81b57a8e912d26865b2f48fdcfa..f711c6a91bcdc8956a756f10ee9af3bba8684b35 100644
--- a/ui/compositor/layer_owner.h
+++ b/ui/compositor/layer_owner.h
@@ -20,7 +20,7 @@ class COMPOSITOR_EXPORT LayerOwner {
   LayerOwner();
   virtual ~LayerOwner();
 
-  void SetLayer(Layer* layer);
+  void SetLayer(std::unique_ptr<Layer> layer);
 
   // Releases the owning reference to its layer, and returns it.
   // This is used when you need to animate the presentation of the owner just
@@ -44,11 +44,11 @@ class COMPOSITOR_EXPORT LayerOwner {
     layer_owner_delegate_ = delegate;
   }
 
+  bool OwnsLayer() const;
+
  protected:
   void DestroyLayer();
 
-  bool OwnsLayer() const;
-
  private:
   // The LayerOwner owns its layer unless ownership is relinquished via a call
   // to AcquireLayer(). After that moment |layer_| will still be valid but