Don't attempt to forward declare StringPiece.

content/public/common/content_client.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 CONTENT_PUBLIC_COMMON_CONTENT_CLIENT_H_
 #define CONTENT_PUBLIC_COMMON_CONTENT_CLIENT_H_
 #pragma once
 
 #include <string>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/string16.h"
+#include "base/string_piece.h"
 #include "build/build_config.h"
 #include "content/common/content_export.h"
 
 class CommandLine;
 class GURL;
 
 namespace IPC {
 class Message;
 }
 
-namespace base {
-class StringPiece;
-}
-
 namespace sandbox {
 class TargetPolicy;
 }
 
 namespace webkit {
 namespace npapi {
 class PluginList;
 }
 }
 
@@ skipping 96 matching lines @@
   ContentPluginClient* plugin_;
   // The embedder API for participating in renderer logic.
   ContentRendererClient* renderer_;
   // The embedder API for participating in utility logic.
   ContentUtilityClient* utility_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_COMMON_CONTENT_CLIENT_H_