Use a direct include of strings headers in chrome/browser/ui/b*-p*/.

chrome/browser/ui/pdf/open_pdf_in_reader_prompt_delegate.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 CHROME_BROWSER_UI_PDF_OPEN_PDF_IN_READER_PROMPT_DELEGATE_H_
 #define CHROME_BROWSER_UI_PDF_OPEN_PDF_IN_READER_PROMPT_DELEGATE_H_
 
-#include "base/string16.h"
+#include "base/strings/string16.h"
 
 namespace content {
 struct LoadCommittedDetails;
 }
 
 class OpenPDFInReaderPromptDelegate {
  public:
   virtual ~OpenPDFInReaderPromptDelegate() {}
 
   virtual string16 GetMessageText() const = 0;
 
   virtual string16 GetAcceptButtonText() const = 0;
 
   virtual string16 GetCancelButtonText() const = 0;
 
   virtual bool ShouldExpire(
       const content::LoadCommittedDetails& details) const = 0;
 
   virtual void Accept() = 0;
 
   virtual void Cancel() = 0;
 };
 
 #endif  // CHROME_BROWSER_UI_PDF_OPEN_PDF_IN_READER_PROMPT_DELEGATE_H_