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

components/scheduler/scheduler.gypi

Index: components/scheduler/scheduler.gypi
diff --git a/components/scheduler/scheduler.gypi b/components/scheduler/scheduler.gypi
index d9f5e38e39e5f43b5c4c65bdc2487cf8f4a729cf..9c4f19c50dae1db22b725db128c8657551460992 100644
--- a/components/scheduler/scheduler.gypi
+++ b/components/scheduler/scheduler.gypi
@@ -98,6 +98,8 @@
       # 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',
     ],
   },
 }