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

base/gfx/bitmap_platform_device.h

Index: base/gfx/bitmap_platform_device.h
===================================================================
--- base/gfx/bitmap_platform_device.h	(revision 1835)
+++ base/gfx/bitmap_platform_device.h	(working copy)
@@ -6,17 +6,25 @@
 // that can be used by upper-level classes that just need to pass a reference
 // around.
 
+#if defined(OS_WIN)
+#include "base/gfx/bitmap_platform_device_win.h"
 namespace gfx {
 
-#if defined(OS_WIN)
-class BitmapPlatformDeviceWin;
 typedef BitmapPlatformDeviceWin BitmapPlatformDevice;
+
+}
 #elif defined(OS_MACOSX)
-class BitmapPlatformDeviceMac;
+#include "base/gfx/bitmap_platform_device_mac.h"
+namespace gfx {
+
 typedef BitmapPlatformDeviceMac BitmapPlatformDevice;
+
+}
 #elif defined(OS_LINUX)
-class BitmapPlatformDeviceLinux;
+#include "base/gfx/bitmap_platform_device_linux.h"
+namespace gfx {
+
 typedef BitmapPlatformDeviceLinux BitmapPlatformDevice;
+
+}
 #endif
-
-}  // namespace gfx