[UI-side compositing] Add lock for accessing NSScrollerImpPair in ScrollerPairMac
<rdar://problem/106652775>
Pull request: https://github.com/WebKit/WebKit/pull/11572
Committed 261759@main (c936c69128b0): <https://commits.webkit.org/261759@main> Reviewed commits have been landed. Closing PR #11572 and removing active labels.