Add testRunner.runIdleTasks() to force idle tasks to run.

components/scheduler/scheduler.gypi

 # 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.
 
 {
   'variables': {
     'scheduler_common_sources': [
       # Sources list duplicated in GN build.
       'common/scheduler_switches.cc',
       'common/scheduler_switches.h',
@@ skipping 80 matching lines @@
       'renderer/user_model.cc',
       'renderer/user_model.h',
       'renderer/webthread_impl_for_renderer_scheduler.cc',
       'renderer/webthread_impl_for_renderer_scheduler.h',
       'scheduler_export.h',
     ],
     'scheduler_test_support_sources': [
       # Sources list duplicated in GN build.
       'test/lazy_scheduler_message_loop_delegate_for_tests.cc',
       'test/lazy_scheduler_message_loop_delegate_for_tests.h',
+      'test/renderer_scheduler_test_support.h',
+      'test/renderer_scheduler_test_support.cc',
     ],
   },
 }