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

chrome/test/base/tracing.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 "chrome/test/base/tracing.h"
 
 #include "base/debug/trace_event.h"
 #include "base/memory/singleton.h"
 #include "base/message_loop.h"
 #include "chrome/test/base/ui_test_utils.h"
@@ skipping 33 matching lines @@
 
     *json_trace_output = output.json_output;
     return true;
   }
 
  private:
   friend struct DefaultSingletonTraits<InProcessTraceController>;
 
   // TraceSubscriber
   virtual void OnEndTracingComplete() OVERRIDE {
-    MessageLoopForUI::current()->Quit();
+    MessageLoopForUI::current()->QuitNow();
   }
 
   // TraceSubscriber
   virtual void OnTraceDataCollected(
       const scoped_refptr<base::RefCountedString>& trace_fragment) OVERRIDE {
     trace_buffer_.AddFragment(trace_fragment->data());
   }
 
   // For collecting trace data asynchronously.
   base::debug::TraceResultBuffer trace_buffer_;
 
   DISALLOW_COPY_AND_ASSIGN(InProcessTraceController);
 };
 
 }  // namespace
 
 namespace tracing {
 
 bool BeginTracing(const std::string& categories) {
   return InProcessTraceController::GetInstance()->BeginTracing(categories);
 }
 
 bool EndTracing(std::string* json_trace_output) {
   return InProcessTraceController::GetInstance()->EndTracing(json_trace_output);
 }
 
 }  // namespace tracing