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

base/message_loop.cc

Index: base/message_loop.cc
diff --git a/base/message_loop.cc b/base/message_loop.cc
index a207659a4a922203a2e3850bbcdde4b1425dd21d..4d6dbb8a3f7368d937d3b3107bd52e9999138fd8 100644
--- a/base/message_loop.cc
+++ b/base/message_loop.cc
@@ -335,11 +335,20 @@ static void QuitCurrent() {
   MessageLoop::current()->Quit();
 }
 
+static void QuitNowCurrent() {
+  MessageLoop::current()->QuitNow();
+}
+
 // static
 base::Closure MessageLoop::QuitClosure() {
   return base::Bind(&QuitCurrent);
 }
 
+// static
+base::Closure MessageLoop::QuitNowClosure() {
+  return base::Bind(&QuitNowCurrent);
+}
+
 void MessageLoop::SetNestableTasksAllowed(bool allowed) {
   if (nestable_tasks_allowed_ != allowed) {
     nestable_tasks_allowed_ = allowed;