Use proper namespace in skia/ext

ui/gfx/image/image_unittest_util_mac.mm

 // Copyright (c) 2012 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.
 
 #import <AppKit/AppKit.h>
 
 #include "skia/ext/skia_utils_mac.h"
 #include "ui/gfx/image/image_unittest_util.h"
 
 namespace gfx {
 namespace test {
 
 SkColor GetPlatformImageColor(PlatformImage image, int x, int y) {
   // AppKit's coordinate system is flipped.
   y = [image size].height - y;
 
   [image lockFocus];
   NSColor* color = NSReadPixel(NSMakePoint(x, y));
   [image unlockFocus];
-  return NSDeviceColorToSkColor(color);
+  return skia::NSDeviceColorToSkColor(color);
 }
 
 }  // namespace test
 }  // namespace gfx