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

base/gfx/bitmap_platform_device_mac.h

Index: base/gfx/bitmap_platform_device_mac.h
===================================================================
--- base/gfx/bitmap_platform_device_mac.h	(revision 1835)
+++ base/gfx/bitmap_platform_device_mac.h	(working copy)
@@ -65,7 +65,8 @@
   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);