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

chrome/browser/ui/login/login_model.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 CHROME_BROWSER_UI_LOGIN_LOGIN_MODEL_H_
 #define CHROME_BROWSER_UI_LOGIN_LOGIN_MODEL_H_
 
-#include "base/string16.h"
+#include "base/strings/string16.h"
 
 // Simple Model & Observer interfaces for a LoginView to facilitate exchanging
 // information.
 class LoginModelObserver {
  public:
   // Called by the model when a username,password pair has been identified
   // as a match for the pending login prompt.
   virtual void OnAutofillDataAvailable(const string16& username,
                                        const string16& password) = 0;
 
   virtual void OnLoginModelDestroying() = 0;
 
  protected:
   virtual ~LoginModelObserver() {}
 };
 
 class LoginModel {
  public:
   // Set the observer interested in the data from the model.
   // observer can be null, signifying there is no longer any observer
   // interested in the data.
   virtual void SetObserver(LoginModelObserver* observer) = 0;
 
  protected:
   virtual ~LoginModel() {}
 };
 
 #endif  // CHROME_BROWSER_UI_LOGIN_LOGIN_MODEL_H_