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

ash/drag_drop/drag_drop_controller.h

Index: ash/drag_drop/drag_drop_controller.h
diff --git a/ash/drag_drop/drag_drop_controller.h b/ash/drag_drop/drag_drop_controller.h
index d44ba20ec51f6161ac6840c3039e73394f2229a3..df13d15470bea45376a1576df37d4a1fe9757ecb 100644
--- a/ash/drag_drop/drag_drop_controller.h
+++ b/ash/drag_drop/drag_drop_controller.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include "ash/ash_export.h"
+#include "base/callback.h"
 #include "ui/aura/client/drag_drop_client.h"
 #include "ui/aura/event.h"
 #include "ui/aura/event_filter.h"
@@ -99,6 +100,9 @@ class ASH_EXPORT DragDropController
   // Only be used for tests.
   bool should_block_during_drag_drop_;
 
+  // Closure for quitting nested message loop.
+  base::Closure quit_closure_;
+
   DISALLOW_COPY_AND_ASSIGN(DragDropController);
 };