Don't attempt to forward declare StringPiece.

ui/base/resource/data_pack.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.
 
 // DataPack represents a read-only view onto an on-disk file that contains
 // (key, value) pairs of data.  It's used to store static resources like
 // translation strings and images.
 
 #ifndef UI_BASE_RESOURCE_DATA_PACK_H_
 #define UI_BASE_RESOURCE_DATA_PACK_H_
 #pragma once
 
 #include <map>
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/string_piece.h"
 #include "ui/base/ui_export.h"
 
 class FilePath;
 class RefCountedStaticMemory;
 
-namespace base {
-class StringPiece;
-}
-
 namespace file_util {
 class MemoryMappedFile;
 }
 
 namespace ui {
 
 class UI_EXPORT DataPack {
  public:
   // What type of encoding the text resources use.
   enum TextEncodingType {
@@ skipping 38 matching lines @@
 
   // Type of encoding for text resources.
   TextEncodingType text_encoding_type_;
 
   DISALLOW_COPY_AND_ASSIGN(DataPack);
 };
 
 }  // namespace ui
 
 #endif  // UI_BASE_RESOURCE_DATA_PACK_H_