Now that the memory regression in PLUM3 has been fixed in the bug 262358, re-enable requestIdleCallback in Apple's ports.
Pull request: https://github.com/WebKit/WebKit/pull/18560
Committed 268800@main (bf33b61e545e): <https://commits.webkit.org/268800@main> Reviewed commits have been landed. Closing PR #18560 and removing active labels.
<rdar://problem/116415761>
Re-opened since this is blocked by bug 262630
Reopening now that PLUM regression is no more.