1311425 - Make requestIdleCallback aware of timeouts.

Prepare for handling several sources of idleness.

Make idle callbacks aware of nsITimers.

Add pref for how far into the timer queue.

1311425 - Avoid expiration timers when scheduling idle runnables.

review comment fix to ensure low priority timers aren't taken into account when calling NS_GetTimerDeadlineHintOnCurrentThread.

xpcom/threads/MainThreadIdlePeriod.h

@@ -16,10 +16,18 @@ class MainThreadIdlePeriod final : public IdlePeriod
 public:
   NS_DECL_NSIIDLEPERIOD
 
+  MainThreadIdlePeriod()
+    : mLastIdleDeadline(TimeStamp::Now())
+  {
+  }
+
   static float GetLongIdlePeriod();
   static float GetMinIdlePeriod();
+  static uint32_t GetMaxTimerThreadBound();
 private:
   virtual ~MainThreadIdlePeriod() {}
+
+  TimeStamp mLastIdleDeadline;
 };
 
 } // namespace mozilla