When a timer is about to fire, idle callback should not have a higher precedence over the timer.
<rdar://problem/56891067>
This is implemented as a part of https://github.com/WebKit/WebKit/pull/16417. Will add a test.
Pull request: https://github.com/WebKit/WebKit/pull/16814
Committed 267029@main (f92900fd3d98): <https://commits.webkit.org/267029@main> Reviewed commits have been landed. Closing PR #16814 and removing active labels.