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

ui/views/controls/menu/menu_controller.cc

Index: ui/views/controls/menu/menu_controller.cc
diff --git a/ui/views/controls/menu/menu_controller.cc b/ui/views/controls/menu/menu_controller.cc
index 0ff715be719e98ba7dde29fe2a95e2871f8cfe7a..76e3c0ec313d0faf45dcebb0ce3fbd2e35669b79 100644
--- a/ui/views/controls/menu/menu_controller.cc
+++ b/ui/views/controls/menu/menu_controller.cc
@@ -6,6 +6,7 @@
 
 #include "base/i18n/case_conversion.h"
 #include "base/i18n/rtl.h"
+#include "base/run_loop.h"
 #include "base/time.h"
 #include "base/utf_string_conversions.h"
 #include "ui/base/dragdrop/drag_utils.h"
@@ -332,7 +333,8 @@ MenuItemView* MenuController::Run(Widget* parent,
   {
     MessageLoopForUI* loop = MessageLoopForUI::current();
     MessageLoop::ScopedNestableTaskAllower allow(loop);
-    loop->RunWithDispatcher(this);
+    base::RunLoop run_loop(this);
+    run_loop.Run();
   }
 #endif
   message_loop_depth_--;