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

base/gfx/platform_canvas.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.
 
 // Declare a platform-neutral name for this platform's canvas class
 // that can be used by upper-level classes that just need to pass a reference
 // around.
 
 #include "build/build_config.h"
 
+#if defined(OS_WIN)
+#include "base/gfx/platform_canvas_win.h"
 namespace gfx {
 
-#if defined(OS_WIN)
-class PlatformCanvasWin;
 typedef PlatformCanvasWin PlatformCanvas;
-#elif defined(OS_MACOSX)
-class PlatformCanvasMac;
-typedef PlatformCanvasMac PlatformCanvas;
-#elif defined(OS_LINUX)
-class PlatformCanvasLinux;
-typedef PlatformCanvasLinux PlatformCanvas;
-#endif
 
 }  // namespace gfx
+#elif defined(OS_MACOSX)
+#include "base/gfx/platform_canvas_mac.h"
+namespace gfx {
+
+typedef PlatformCanvasMac PlatformCanvas;
+
+}  // namespace gfx
+#elif defined(OS_LINUX)
+#include "base/gfx/platform_canvas_linux.h"
+namespace gfx {
+
+typedef PlatformCanvasLinux PlatformCanvas;
+
+}  // namespace gfx
+#endif