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

base/message_loop.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_LOOP_H_
 #define BASE_MESSAGE_LOOP_H_
 #pragma once
 
 #include <queue>
 #include <string>
@@ skipping 224 matching lines @@
   void Quit();
 
   // This method is a variant of Quit, that does not wait for pending messages
   // to be processed before returning from Run.
   void QuitNow();
 
   // Invokes Quit on the current MessageLoop when run. Useful to schedule an
   // arbitrary MessageLoop to Quit.
   static base::Closure QuitClosure();
 
+  // Invokes QuitNow on the current MessageLoop when run. Useful to schedule an
+  // arbitrary MessageLoop to QuitNow.
+  static base::Closure QuitNowClosure();
+
   // Returns the type passed to the constructor.
   Type type() const { return type_; }
 
   // Optional call to connect the thread name with this loop.
   void set_thread_name(const std::string& thread_name) {
     DCHECK(thread_name_.empty()) << "Should not rename this thread!";
     thread_name_ = thread_name;
   }
   const std::string& thread_name() const { return thread_name_; }
 
@@ skipping 404 matching lines @@
 #endif  // defined(OS_POSIX)
 };
 
 // Do not add any member variables to MessageLoopForIO!  This is important b/c
 // MessageLoopForIO is often allocated via MessageLoop(TYPE_IO).  Any extra
 // data that you need should be stored on the MessageLoop's pump_ instance.
 COMPILE_ASSERT(sizeof(MessageLoop) == sizeof(MessageLoopForIO),
                MessageLoopForIO_should_not_have_extra_member_variables);
 
 #endif  // BASE_MESSAGE_LOOP_H_