Use a direct include of strings headers in content/p*/.

content/public/browser/web_ui_message_handler.h

 // 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.
 
 #ifndef CONTENT_PUBLIC_BROWSER_WEB_UI_MESSAGE_HANDLER_H_
 #define CONTENT_PUBLIC_BROWSER_WEB_UI_MESSAGE_HANDLER_H_
 
 #include "base/basictypes.h"
 #include "base/gtest_prod_util.h"
-#include "base/string16.h"
+#include "base/strings/string16.h"
 #include "content/common/content_export.h"
 
 class GURL;
 class WebUIBrowserTest;
 
 namespace base {
 class DictionaryValue;
 class ListValue;
 }
 
@@ skipping 42 matching lines @@
   // Provide external classes access to web_ui() and set_web_ui().
   friend class WebUIImpl;
   friend class ::WebUIBrowserTest;
 
   WebUI* web_ui_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_WEB_UI_MESSAGE_HANDLER_H_