Use modern base APIs to avoid base::MessageLoop::current() calls in /ui/views

This is a precusor cleanup extracted from
https://chromium-review.googlesource.com/c/chromium/src/+/957760/7
in the broader effort to restrict usage of MessageLoop::current().

This CL was uploaded by git cl split.

R=msw@chromium.org

Bug: 825327
Change-Id: I7d3d65d961089d61bacc89b5de1d526ea0cf94fc
Reviewed-on: https://chromium-review.googlesource.com/986756
Commit-Queue: Michael Wasserman <msw@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546965}

ui/views/widget/widget_interactive_uitest.cc

@@ -377,9 +377,7 @@ class TouchEventHandler : public ui::EventHandler {
   }
 
   void WaitForEvents() {
-    base::MessageLoopForUI* loop = base::MessageLoopForUI::current();
-    base::MessageLoopForUI::ScopedNestableTaskAllower allow_nested(loop);
-    base::RunLoop run_loop;
+    base::RunLoop run_loop(base::RunLoop::Type::kNestableTasksAllowed);
     quit_closure_ = run_loop.QuitClosure();
     run_loop.Run();
   }