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..44cd28cb9dca58f791d39415ac4f8ce9e40c57f6 100644
--- a/chrome/test/perf/rendering/throughput_tests.cc
+++ b/chrome/test/perf/rendering/throughput_tests.cc
@@ -8,6 +8,7 @@
 #include "base/json/json_reader.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/path_service.h"
+#include "base/run_loop.h"
 #include "base/string_number_conversions.h"
 #include "base/stringprintf.h"
 #include "base/test/trace_event_analyzer.h"
@@ -195,11 +196,11 @@ class ThroughputTest : public BrowserPerfTest {
   }
 
   void Wait(int ms) {
+    base::RunLoop run_loop;
     MessageLoop::current()->PostDelayedTask(
-        FROM_HERE,
-        MessageLoop::QuitClosure(),
+        FROM_HERE, run_loop.QuitClosure(),
         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.