ash: Do not use MessageLoopForUI when not needed.

ash/shell/content/client/shell_browser_main_parts.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 "ash/shell/content/client/shell_browser_main_parts.h"
 
 #include "ash/ash_switches.h"
 #include "ash/content/shell_content_state.h"
 #include "ash/desktop_background/desktop_background_controller.h"
 #include "ash/shell.h"
@@ skipping 152 matching lines @@
   views_delegate_.reset();
 
   // The keyboard may have created a WebContents. The WebContents is destroyed
   // with the UI, and it needs the BrowserContext to be alive during its
   // destruction. So destroy all of the UI elements before destroying the
   // browser context.
   browser_context_.reset();
 }
 
 bool ShellBrowserMainParts::MainMessageLoopRun(int* result_code) {
-  base::MessageLoopForUI::current()->Run();
+  base::MessageLoop::current()->Run();
   return true;
 }
 
 }  // namespace shell
 }  // namespace ash