Generalize layer mirroring for phantom windows

ash/common/wm/forwarding_layer_delegate.h

Index: ash/common/wm/forwarding_layer_delegate.h
diff --git a/ash/common/wm/forwarding_layer_delegate.h b/ash/common/wm/forwarding_layer_delegate.h
index 7c68f185a10d658c7b92e6e7f58c15bc8fcb7d9c..023cb53aa8875e3a88bb2f11b632fa6ac0d13188 100644
--- a/ash/common/wm/forwarding_layer_delegate.h
+++ b/ash/common/wm/forwarding_layer_delegate.h
@@ -15,9 +15,6 @@ class Layer;
 }
 
 namespace ash {
-
-class WmWindow;
-
 namespace wm {
 
 // A layer delegate to paint the content of a recreated layer by delegating
@@ -36,14 +33,13 @@ class ForwardingLayerDelegate : public ui::LayerDelegate,
 
   // ui::LayerObserver:
   void DidPaintLayer(ui::Layer* layer, const gfx::Rect& rect) override;
-  void SurfaceChanged(ui::Layer* layer) override;
   void LayerDestroyed(ui::Layer* layer) override;
 
  private:
   // The layer for which |this| is the delegate.
-  ui::Layer* client_layer_;
+  ui::Layer* layer_;
 
-  // The layer that was recreated to replace |new_layer_|.
+  // The layer to which paint requests are forwarded.
   ui::Layer* original_layer_;
 
   ScopedObserver<ui::Layer, ui::LayerObserver> scoped_observer_;