ui: Cleanup class/struct forward declarations

ui/compositor/layer_delegate.h

 // Copyright (c) 2012 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_DELEGATE_H_
 #define UI_COMPOSITOR_LAYER_DELEGATE_H_
 
 #include "ui/compositor/compositor_export.h"
 
 namespace gfx {
-class Canvas;
 class Rect;
 }
 
 namespace ui {
 class PaintContext;
 
 // A delegate interface implemented by an object that renders to a Layer.
 class COMPOSITOR_EXPORT LayerDelegate {
  public:
   // Paint content for the layer to the specified context.
   virtual void OnPaintLayer(const PaintContext& context) = 0;
 
   // A notification that this layer has had a delegated frame swap and
   // will be repainted.
   virtual void OnDelegatedFrameDamage(const gfx::Rect& damage_rect_in_dip) = 0;
 
   // Called when the layer's device scale factor has changed.
   virtual void OnDeviceScaleFactorChanged(float device_scale_factor) = 0;
 
   // Invoked when the bounds have changed.
   virtual void OnLayerBoundsChanged(const gfx::Rect& old_bounds);
 
  protected:
   virtual ~LayerDelegate() {}
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_LAYER_DELEGATE_H_