Use a standard message box for DesktopCastingWarningView.

chrome/browser/ui/cocoa/simple_message_box_mac.mm

 // 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/simple_message_box.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/mac/scoped_nsobject.h"
 #include "base/strings/sys_string_conversions.h"
@@ skipping 66 matching lines @@
   return false;
 }
 
 MessageBoxResult ShowQuestionMessageBox(gfx::NativeWindow parent,
                                         const base::string16& title,
                                         const base::string16& message) {
   return ShowMessageBox(parent, title, message, base::string16(),
                         MESSAGE_BOX_TYPE_QUESTION);
 }
 
+bool CloseMessageBoxForTest(bool accept) {
+  NOTIMPLEMENTED();
+  return false;
+}
+
 }  // namespace chrome