Use dynamicDowncast<T> in AccessibilityARIAGridRow.cpp
<rdar://problem/120556207>
Pull request: https://github.com/WebKit/WebKit/pull/22443
Committed 272824@main (208e146cb95a): <https://commits.webkit.org/272824@main> Reviewed commits have been landed. Closing PR #22443 and removing active labels.