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 3e218ef5a4a7fc127fc138521ba31975ffdb1f36..dbcfefd96f2a4598d6601925e9cfd3a4812af032 100644
--- a/components/scheduler/scheduler.gypi
+++ b/components/scheduler/scheduler.gypi
@@ -97,6 +97,8 @@
       # 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',
     ],
   },
 }