Use ScopedTaskEnvironment instead of MessageLoopForUI in ui/aura/test/.

ui/aura/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 
 component("aura") {
   sources = [
     "../wm/public/activation_change_observer.cc",
@@ skipping 258 matching lines @@
     "test/window_test_api.h",
   ]
 
   # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
   configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 
   public_deps = [
     ":aura",
   ]
   deps = [
+    "//base/test:test_support",
     "//services/ui/public/interfaces",
     "//skia",
     "//testing/gtest",
     "//ui/base:test_support",
     "//ui/base/ime",
     "//ui/compositor:test_support",
     "//ui/display",
     "//ui/events",
     "//ui/events:events_base",
     "//ui/events:test_support",
@@ skipping 91 matching lines @@
   ]
 
   if (is_linux) {
     deps += [ "//third_party/mesa" ]
   }
 
   data_deps = [
     "//third_party/mesa:osmesa",
   ]
 }