WebXRSession should not hold a unique ptr to WebXRViewerSpace which is ref counted
<rdar://problem/115180471>
Pull request: https://github.com/WebKit/WebKit/pull/19995
Committed 270230@main (67e35fd334d5): <https://commits.webkit.org/270230@main> Reviewed commits have been landed. Closing PR #19995 and removing active labels.