Factor out uses of the WebContentsModalDialog interface from platform-independent code

chrome/browser/ui/android/login_prompt_android.cc

 // 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.
 
 #include "chrome/browser/ui/login/login_prompt.h"
 
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/string16.h"
 #include "base/utf_string_conversions.h"
@@ skipping 52 matching lines @@
     } else {
       CancelAuth();
       LOG(WARNING) << "HTTP Authentication failed because TabAndroid is "
           "missing";
     }
   }
 
  protected:
   virtual ~LoginHandlerAndroid() {}
 
+  virtual void CloseDialog() OVERRIDE {}
+
  private:
   scoped_ptr<ChromeHttpAuthHandler> chrome_http_auth_handler_;
 };
 
 // static
 LoginHandler* LoginHandler::Create(net::AuthChallengeInfo* auth_info,
                                    net::URLRequest* request) {
   return new LoginHandlerAndroid(auth_info, request);
 }