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

base/message_loop.h

Index: base/message_loop.h
diff --git a/base/message_loop.h b/base/message_loop.h
index d78b58addc2019ad46b61e79adf8c918f432a9dd..2dfb85c1549ca092b92c1bb01a3864b4154ca189 100644
--- a/base/message_loop.h
+++ b/base/message_loop.h
@@ -242,6 +242,10 @@ class BASE_EXPORT MessageLoop : public base::MessagePump::Delegate {
   // 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_; }