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

chrome/test/base/tracing.cc

Index: chrome/test/base/tracing.cc
diff --git a/chrome/test/base/tracing.cc b/chrome/test/base/tracing.cc
index 790ab3c8016dde6ea53f0af9b6a6c8da843fd88c..3c29bba32366d94e776969d712e85d98ce62d7dc 100644
--- a/chrome/test/base/tracing.cc
+++ b/chrome/test/base/tracing.cc
@@ -38,7 +38,8 @@ class InProcessTraceController : public content::TraceSubscriber {
       return false;
     // Wait for OnEndTracingComplete() to quit the message loop.
     // OnTraceDataCollected may be called multiple times while blocking here.
-    ui_test_utils::RunMessageLoop();
+    message_loop_runner_ = new ui_test_utils::MessageLoopRunner;
+    message_loop_runner_->Run();
     trace_buffer_.Finish();
     trace_buffer_.SetOutputCallback(TraceResultBuffer::OutputCallback());
 
@@ -51,7 +52,7 @@ class InProcessTraceController : public content::TraceSubscriber {
 
   // TraceSubscriber
   virtual void OnEndTracingComplete() OVERRIDE {
-    MessageLoopForUI::current()->Quit();
+    message_loop_runner_->Quit();
   }
 
   // TraceSubscriber
@@ -63,6 +64,8 @@ class InProcessTraceController : public content::TraceSubscriber {
   // For collecting trace data asynchronously.
   base::debug::TraceResultBuffer trace_buffer_;
 
+  scoped_refptr<ui_test_utils::MessageLoopRunner> message_loop_runner_;
+
   DISALLOW_COPY_AND_ASSIGN(InProcessTraceController);
 };