Introduce ThreadTaskRunnerHandle::OverrideForTesting and TestMockTimeTaskRunner::ScopedContext.

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 2106 matching lines @@
     "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",
     "threading/simple_thread_unittest.cc",
     "threading/thread_checker_unittest.cc",
     "threading/thread_collision_warner_unittest.cc",
     "threading/thread_id_name_manager_unittest.cc",
     "threading/thread_local_storage_unittest.cc",
     "threading/thread_local_unittest.cc",
+    "threading/thread_task_runner_handle_unittest.cc",
     "threading/thread_unittest.cc",
     "threading/watchdog_unittest.cc",
     "threading/worker_pool_posix_unittest.cc",
     "threading/worker_pool_unittest.cc",
     "time/pr_time_unittest.cc",
     "time/time_unittest.cc",
     "time/time_win_unittest.cc",
     "timer/hi_res_timer_manager_unittest.cc",
     "timer/mock_timer_unittest.cc",
     "timer/timer_unittest.cc",
@@ skipping 519 matching lines @@
 }
 
 fuzzer_test("base_json_correctness_fuzzer") {
   sources = [
     "json/correctness_fuzzer.cc",
   ]
   deps = [
     ":base",
   ]
 }