[UI-side compositing] Have pinch zoom constrain zoom origin in commitTransientZoom
<rdar://problem/105515035>
Pull request: https://github.com/WebKit/WebKit/pull/10173
Committed 261226@main (d1ac2b0c842b): <https://commits.webkit.org/261226@main> Reviewed commits have been landed. Closing PR #10173 and removing active labels.