investigate window.requestIdleCallback() for possible timing leaks

The window.requestIdleCallback() API is available as of Firefox 55. We should determine whether it may be used to learn too much about the performance of the user's computer/device, or if there are other timing leaks we want to avoid. See: ​ https://bugzilla.mozilla.org/show_bug.cgi?id=1314959 ​ https://developer.mozilla.org/en-US/docs/Web/API/Background_Tasks_API

If necessary, we can disable this feature by setting dom.requestIdleCallback.enabled to false.