Bug 1462784 - Add idle annotations for DOM worker threads. r=froydnj

MozReview-Commit-ID: 7WM1ALzhDzY

--HG--
extra : rebase_source : 93f1bea71694992e4d344bca6a272abf0e66f719

dom/workers/WorkerPrivate.cpp

@@ -3823,6 +3823,8 @@ WorkerPrivate::DisableMemoryReporter()
 void
 WorkerPrivate::WaitForWorkerEvents()
 {
+  AUTO_PROFILER_LABEL("WorkerPrivate::WaitForWorkerEvents", IDLE);
+
   AssertIsOnWorkerThread();
   mMutex.AssertCurrentThreadOwns();