Don't attempt to forward declare StringPiece.

ui/base/resource/resource_bundle.h

 // Copyright (c) 2011 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.
 
 #ifndef UI_BASE_RESOURCE_RESOURCE_BUNDLE_H_
 #define UI_BASE_RESOURCE_RESOURCE_BUNDLE_H_
 #pragma once
 
 #include "build/build_config.h"
 
 #if defined(OS_WIN)
 #include <windows.h>
 #endif
 
 #include <map>
 #include <string>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/file_path.h"
 #include "base/gtest_prod_util.h"
 #include "base/memory/ref_counted_memory.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/string16.h"
+#include "base/string_piece.h"
 #include "ui/base/ui_export.h"
 #include "ui/gfx/native_widget_types.h"
 
 class SkBitmap;
 typedef uint32 SkColor;
 
 namespace base {
 class Lock;
-class StringPiece;
 }
 
 namespace gfx {
 class Font;
 class Image;
 }
 
 #if defined(OS_MACOSX)
 #ifdef __OBJC__
 @class NSImage;
@@ skipping 271 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(ResourceBundle);
 };
 
 }  // namespace ui
 
 // TODO(beng): Someday, maybe, get rid of this.
 using ui::ResourceBundle;
 
 #endif  // UI_BASE_RESOURCE_RESOURCE_BUNDLE_H_