Refactor the platform canvas unit test a bit so that it can be run on...

base/gfx/bitmap_platform_device_mac.h

 // Copyright (c) 2006-2008 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 BASE_GFX_BITMAP_PLATFORM_DEVICE_MAC_H__
 #define BASE_GFX_BITMAP_PLATFORM_DEVICE_MAC_H__
 
 #include "base/gfx/platform_device_mac.h"
 #include "base/ref_counted.h"
 
@@ skipping 47 matching lines @@
   virtual CGContextRef GetBitmapContext();
   virtual void SetTransform(const SkMatrix& matrix);
   virtual void SetDeviceOffset(int x, int y);
 
   // This currently only supports extremely simple clip rects.
   virtual void SetClipRegion(const SkRegion& region);
 
   virtual void DrawToContext(CGContextRef context, int x, int y,
                              const CGRect* src_rect);
   virtual bool IsVectorial() { return false; }
-
+  virtual void fixupAlphaBeforeCompositing() { };
+  
   // Returns the color value at the specified location. This does not
   // consider any transforms that may be set on the device.
   SkColor GetColorAt(int x, int y);
 
  protected:
   // Reference counted data that can be shared between multiple devices. This
   // allows copy constructors and operator= for devices to work properly. The
   // bitmaps used by the base device class are already refcounted and copyable.
   class BitmapPlatformDeviceMacData;
 
@@ skipping 10 matching lines @@
 
   virtual void processPixels(int x, int y,
                              int width, int height, 
                              adjustAlpha adjustor);
 };
 
 }  // namespace gfx
 
 #endif  // BASE_GFX_BITMAP_PLATFORM_DEVICE_MAC_H__