Use ScopedTaskEnvironment instead of MessageLoopForUI in remoting tests.

remoting/test/BUILD.gn

 # Copyright 2015 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("//remoting/remoting_options.gni")
 
 static_library("test_support") {
   testonly = true
 
   sources = [
@@ skipping 155 matching lines @@
   static_library("it2me_standalone_host") {
     testonly = true
 
     sources = [
       "it2me_standalone_host.cc",
       "it2me_standalone_host.h",
     ]
 
     deps = [
       ":fake_connection_event_logger",
+      "//base/test:test_support",
       "//third_party/webrtc/modules/desktop_capture",
     ]
 
     public_deps = [
       "//remoting/host:test_support",
       "//testing/gmock",
     ]
   }
 
   executable("it2me_standalone_host_main") {
@@ skipping 43 matching lines @@
     ":ar_test_driver_common",
     ":test_support",
     "//base",
     "//net:test_support",
     "//testing/gmock",
     "//testing/gtest",
     "//third_party/libyuv",
     "//third_party/webrtc/modules/desktop_capture",
   ]
 }