Use a standard message box for DesktopCastingWarningView.

chrome/browser/ui/simple_message_box.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_SIMPLE_MESSAGE_BOX_H_
 #define CHROME_BROWSER_UI_SIMPLE_MESSAGE_BOX_H_
 
 #include "base/strings/string16.h"
 #include "ui/gfx/native_widget_types.h"
 
@@ skipping 43 matching lines @@
 //
 // NOTE: In general, you should avoid this since it's usually poor UI.
 // We have a variety of other surfaces such as app menu notifications and
 // infobars; consult the UI leads for a recommendation.
 MessageBoxResult ShowMessageBoxWithButtonText(gfx::NativeWindow parent,
                                               const base::string16& title,
                                               const base::string16& message,
                                               const base::string16& yes_text,
                                               const base::string16& no_text);
 
+// Closes the current message box, if any, accepting or declining based on
+// |accept|. Returns whether there was a message box showing.
+bool CloseMessageBoxForTest(bool accept);

[Daniel Erat, 2016/10/27 17:23:19]

nit: consider WARN_UNUSED_RESULT if tests ought to always check the return value

[Evan Stade, 2016/10/27 19:11:18]

Done.

+
 }  // namespace chrome
 
 #endif  // CHROME_BROWSER_UI_SIMPLE_MESSAGE_BOX_H_