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.
1367905 - Try to run GC/CC slices, including forgetSkippable, during idle time.
1367905 - Try to run GC/CC slices, including forgetSkippable, during idle time, tweaks to keep reftest memory usage lower.
Since we are using vsync for timing anyway in the layout refresh driver,
there is no need to bump the system timer resolution down to 1 ms.
This resolves#1507.
This creates a number of stubs and leaves some surrounding code that may be irrelevant (eg. recorded time stamps, status variables).
Stub resolution/removal should be a follow-up to this.
