ui: Cleanup class/struct forward declarations

ui/compositor/paint_context.h

 // Copyright 2015 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_PAINT_CONTEXT_H_
 #define UI_COMPOSITOR_PAINT_CONTEXT_H_
 
 #include <memory>
 
 #include "base/logging.h"
 #include "base/macros.h"
 #include "ui/compositor/compositor_export.h"
 #include "ui/gfx/geometry/rect.h"
 
 namespace cc {
 class DisplayItemList;
 }
 
-namespace gfx {
-class Canvas;
-}
-
 class SkPictureRecorder;
 
 namespace ui {
 class ClipRecorder;
 class CompositingRecorder;
 class PaintRecorder;
 class TransformRecorder;
 
 class COMPOSITOR_EXPORT PaintContext {
  public:
@@ skipping 80 matching lines @@
   // recorder is active.
   mutable bool inside_paint_recorder_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(PaintContext);
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_PAINT_CONTEXT_H_