Fix compile error in ScrollingTreeFrameScrollingNodeMac.mm when -Wthread-safety-analysis is enabled
<rdar://problem/100915503>
Pull request: https://github.com/WebKit/WebKit/pull/5156
Committed 255350@main (b243d889a53c): <https://commits.webkit.org/255350@main> Reviewed commits have been landed. Closing PR #5156 and removing active labels.