Fix compilation with MTLSHAREDEVENT_FOR_XR_FRAME_COMPLETION enabled
<rdar://problem/113608823>
Pull request: https://github.com/WebKit/WebKit/pull/16514
Committed 266718@main (2d4c384f00d9): <https://commits.webkit.org/266718@main> Reviewed commits have been landed. Closing PR #16514 and removing active labels.