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

base/gfx/platform_canvas.h

Index: base/gfx/platform_canvas.h
===================================================================
--- base/gfx/platform_canvas.h	(revision 1896)
+++ base/gfx/platform_canvas.h	(working copy)
@@ -8,17 +8,25 @@
 
 #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;
+
+}  // namespace gfx
 #elif defined(OS_MACOSX)
-class PlatformCanvasMac;
+#include "base/gfx/platform_canvas_mac.h"
+namespace gfx {
+
 typedef PlatformCanvasMac PlatformCanvas;
+
+}  // namespace gfx
 #elif defined(OS_LINUX)
-class PlatformCanvasLinux;
+#include "base/gfx/platform_canvas_linux.h"
+namespace gfx {
+
 typedef PlatformCanvasLinux PlatformCanvas;
-#endif
 
 }  // namespace gfx
+#endif