Bug 1311425 - Avoid expiration timers when scheduling idle runnables, r=froydnj

--HG--
extra : rebase_source : b20edf82a69a24965b4bb6300ff9b1be39ad7b8a

xpcom/ds/nsExpirationTracker.h

@@ -423,8 +423,12 @@ private:
       NS_ENSURE_STATE(target);
       mTimer->SetTarget(target);
     }
-    mTimer->InitWithNamedFuncCallback(TimerCallback, this, mTimerPeriod,
-                                      nsITimer::TYPE_REPEATING_SLACK, mName);
+    mTimer->InitWithNamedFuncCallback(
+      TimerCallback,
+      this,
+      mTimerPeriod,
+      nsITimer::TYPE_REPEATING_SLACK_LOW_PRIORITY,
+      mName);
     return NS_OK;
   }
 };