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

base/gfx/platform_device.h

Index: base/gfx/platform_device.h
===================================================================
--- base/gfx/platform_device.h	(revision 1896)
+++ base/gfx/platform_device.h	(working copy)
@@ -6,17 +6,22 @@
 // that can be used by upper-level classes that just need to pass a reference
 // around.
 
+#if defined(OS_WIN)
+#include "base/gfx/platform_device_win.h"
+#elif defined(OS_MACOSX)
+#include "base/gfx/platform_device_mac.h"
+#elif defined(OS_LINUX)
+#include "base/gfx/platform_device_linux.h"
+#endif
+
 namespace gfx {
-
+  
 #if defined(OS_WIN)
-class PlatformDeviceWin;
 typedef PlatformDeviceWin PlatformDevice;
 #elif defined(OS_MACOSX)
-class PlatformDeviceMac;
 typedef PlatformDeviceMac PlatformDevice;
 #elif defined(OS_LINUX)
-class PlatformDeviceLinux;
 typedef PlatformDeviceLinux PlatformDevice;
 #endif
-
+  
 }  // namespace gfx