Add base::RunLoop and update ui_test_utils to use it to reduce flakiness

base/message_pump_dispatcher.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 BASE_MESSAGE_PUMP_DISPATCHER_H
 #define BASE_MESSAGE_PUMP_DISPATCHER_H
 #pragma once
 
 #include "base/base_export.h"
 #include "base/event_types.h"
 
 namespace base {
 
-// Dispatcher is used during a nested invocation of Run to dispatch
-// events when |MessageLoop::RunWithDispatcher| is invoked.  If
-// |MessageLoop::Run| is invoked, MessageLoop does not dispatch events
-// (or invoke TranslateMessage), rather every message is passed to
-// Dispatcher's Dispatch method for dispatch. It is up to the
-// Dispatcher whether or not to dispatch the event.
+// Dispatcher is used during a nested invocation of Run to dispatch events when
+// |RunLoop(dispatcher).Run()| is used.  If |RunLoop().Run()| is invoked,
+// MessageLoop does not dispatch events (or invoke TranslateMessage), rather
+// every message is passed to Dispatcher's Dispatch method for dispatch. It is
+// up to the Dispatcher whether or not to dispatch the event.
 //
 // The nested loop is exited by either posting a quit, or returning false
 // from Dispatch.
 class BASE_EXPORT MessagePumpDispatcher {
  public:
   virtual ~MessagePumpDispatcher() {}
 
   // Dispatches the event. If true is returned processing continues as
   // normal. If false is returned, the nested loop exits immediately.
   virtual bool Dispatch(const NativeEvent& event) = 0;
 };
 
 }  // namespace base
 
 #endif  // BASE_MESSAGE_PUMP_DISPATCHER_H