Bug 1600331. When an idle runnable is queued from a background thread, lazily queue it from a non-idle runnable. r=smaug

Idle runnables do weird things involving unlocking the event queue mutex while
looking for runnables, such that queueing one from off the main thread might
cause it to basically never run if it gets queued during one of those
temporary-unlock periods.

Differential Revision: https://phabricator.services.mozilla.com/D65019