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

chrome/test/perf/rendering/throughput_tests.cc

Index: chrome/test/perf/rendering/throughput_tests.cc
diff --git a/chrome/test/perf/rendering/throughput_tests.cc b/chrome/test/perf/rendering/throughput_tests.cc
index 606fb940c6ac9d8e8cdf9fced8abe3ddc9f20cab..d97829dc581319f68641be72771f7d80be9a8ba7 100644
--- a/chrome/test/perf/rendering/throughput_tests.cc
+++ b/chrome/test/perf/rendering/throughput_tests.cc
@@ -195,11 +195,11 @@ class ThroughputTest : public BrowserPerfTest {
   }
 
   void Wait(int ms) {
+    MessageLoop::RunLoop run_loop;
     MessageLoop::current()->PostDelayedTask(
-        FROM_HERE,
-        MessageLoop::QuitClosure(),
+        FROM_HERE, run_loop.StopClosure(),
         base::TimeDelta::FromMilliseconds(ms));
-    ui_test_utils::RunMessageLoop();
+    ui_test_utils::RunRunLoop(run_loop.AsWeakPtr());
   }
 
   // Take snapshot of the current tab, encode it as PNG, and save to a SkBitmap.