Add base::test::ScopedTaskEnvironment::RunUntilIdle().

base/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.
 
 # HOW TO WRITE CONDITIONALS IN THIS FILE
 # ======================================
 #
 # In many other places, one would write a conditional that expresses all the
 # cases when a source file is used or unused, and then either add or subtract
 # it from the sources list in that case
@@ skipping 2126 matching lines @@
     "task_scheduler/task_tracker_unittest.cc",
     "task_scheduler/task_traits_unittest.cc",
     "task_scheduler/task_unittest.cc",
     "task_scheduler/test_task_factory.cc",
     "task_scheduler/test_task_factory.h",
     "task_scheduler/test_utils.h",
     "template_util_unittest.cc",
     "test/histogram_tester_unittest.cc",
     "test/mock_callback_unittest.cc",
     "test/scoped_mock_time_message_loop_task_runner_unittest.cc",
+    "test/scoped_task_environment_unittest.cc",
     "test/scoped_task_scheduler_unittest.cc",
     "test/test_pending_task_unittest.cc",
     "test/test_reg_util_win_unittest.cc",
     "test/trace_event_analyzer_unittest.cc",
     "test/user_action_tester_unittest.cc",
     "threading/non_thread_safe_unittest.cc",
     "threading/platform_thread_unittest.cc",
     "threading/post_task_and_reply_impl_unittest.cc",
     "threading/sequenced_task_runner_handle_unittest.cc",
     "threading/sequenced_worker_pool_unittest.cc",
@@ skipping 564 matching lines @@
 }
 
 fuzzer_test("base_json_correctness_fuzzer") {
   sources = [
     "json/correctness_fuzzer.cc",
   ]
   deps = [
     ":base",
   ]
 }