Generalize layer mirroring for phantom windows

ui/compositor/layer_owner_delegate.h

Index: ui/compositor/layer_owner_delegate.h
diff --git a/ui/compositor/layer_owner_delegate.h b/ui/compositor/layer_owner_delegate.h
index 68ba92ee445673fb814168fda7812f505473d72a..bf7047ac5a5286cd6be11a2917191a7b46453183 100644
--- a/ui/compositor/layer_owner_delegate.h
+++ b/ui/compositor/layer_owner_delegate.h
@@ -10,14 +10,19 @@
 namespace ui {
 class Layer;
 
-// Called from RecreateLayer() after the new layer was created. old_layer is
-// the layer that will be returned to the caller of RecreateLayer, new_layer
-// will be the layer now used. Used when the layer has external content
-// (SetTextureMailbox / SetDelegatedFrame was called).
+// Used for layers with external content, e.g. a surface via SetShowSurface.
 class COMPOSITOR_EXPORT LayerOwnerDelegate {
  public:
+  // Called from RecreateLayer after the new layer is created. |old_layer| is
+  // the previously owned layer returned to the caller of RecreateLayer, and
+  // |new_layer| is the recreated layer now owned. |new_layer| should share
+  // the content of |old_layer| after this call.
   virtual void OnLayerRecreated(Layer* old_layer, Layer* new_layer) = 0;
 
+  // Called from MirrorLayer. |mirror| should share the content of the owned
+  // layer after this call.
+  virtual void OnLayerMirrored(Layer* mirror) = 0;
+
  protected:
   virtual ~LayerOwnerDelegate() {}
 };