Add base::test::ScopedTaskEnvironment.

base/test/BUILD.gn

 # Copyright (c) 2013 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("//build/config/nacl/config.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
@@ skipping 66 matching lines @@
     "scoped_command_line.cc",
     "scoped_command_line.h",
     "scoped_feature_list.cc",
     "scoped_feature_list.h",
     "scoped_locale.cc",
     "scoped_locale.h",
     "scoped_mock_time_message_loop_task_runner.cc",
     "scoped_mock_time_message_loop_task_runner.h",
     "scoped_path_override.cc",
     "scoped_path_override.h",
+    "scoped_task_environment.cc",
+    "scoped_task_environment.h",
     "scoped_task_scheduler.cc",
     "scoped_task_scheduler.h",
     "sequenced_task_runner_test_template.cc",
     "sequenced_task_runner_test_template.h",
     "sequenced_worker_pool_owner.cc",
     "sequenced_worker_pool_owner.h",
     "simple_test_clock.cc",
     "simple_test_clock.h",
     "simple_test_tick_clock.cc",
     "simple_test_tick_clock.h",
@@ skipping 272 matching lines @@
     testonly = true
     import_include = [
       "android/java/src",
       "//base/android/java/src",
     ]
     sources = [
       "android/java/src/org/chromium/base/ITestClient.aidl",
     ]
   }
 }