Generalize layer mirroring for phantom windows

ui/compositor/layer_observer.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_COMPOSITOR_LAYER_OBSERVER_H_
 #define UI_COMPOSITOR_LAYER_OBSERVER_H_
 
 #include "ui/compositor/compositor_export.h"
 
 namespace ui {
 
 class Layer;
 
+// TODO(domlaskowski): This is only used for layer mirroring, so it should be
+// removed and replaced by a private equivalent in the Layer class.
 class COMPOSITOR_EXPORT LayerObserver {
  public:
-  virtual void DidPaintLayer(Layer* layer, const gfx::Rect& region) {}
-  virtual void SurfaceChanged(Layer* layer) {}
   virtual void LayerDestroyed(Layer* layer) {}
 
  protected:
   virtual ~LayerObserver() {}
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_LAYER_OBSERVER_H_