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

base/gfx/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_PLATFORM_DEVICE_MAC_H__
 #define BASE_GFX_PLATFORM_DEVICE_MAC_H__
 
 #import <ApplicationServices/ApplicationServices.h>
 #include "SkDevice.h"
 
@@ skipping 41 matching lines @@
   // the entirety of the source device will be copied.
   virtual void DrawToContext(CGContextRef context, int x, int y,
                              const CGRect* src_rect) = 0;
 
   // Sets the opacity of each pixel in the specified region to be opaque.
   void makeOpaque(int x, int y, int width, int height);
 
   // Returns if the preferred rendering engine is vectorial or bitmap based.
   virtual bool IsVectorial() = 0;
 
+  // On platforms where the native rendering API does not support rendering
+  // into bitmaps with a premultiplied alpha channel, this call is responsible
+  // for doing any fixup necessary.  It is not used on the Mac, since
+  // CoreGraphics can handle premultiplied alpha just fine.
+  virtual void fixupAlphaBeforeCompositing() = 0;
+
   // Initializes the default settings and colors in a device context.
   static void InitializeCGContext(CGContextRef context);
 
   // Loads a SkPath into the CG context. The path can there after be used for
   // clipping or as a stroke.
   static void LoadPathToCGContext(CGContextRef context, const SkPath& path);
 
  protected:
   // Forwards |bitmap| to SkDevice's constructor.
   PlatformDeviceMac(const SkBitmap& bitmap);
@@ skipping 12 matching lines @@
                              int y,
                              int width,
                              int height,
                              adjustAlpha adjustor) = 0;
 };
 
 }  // namespace gfx
 
 #endif  // BASE_GFX_PLATFORM_DEVICE_MAC_H__