Re-add static assert for scrollable area size
See https://commits.webkit.org/258257@main
<rdar://problem/103763815>
Pull request: https://github.com/WebKit/WebKit/pull/8478
Committed 258888@main (f68ac346edce): <https://commits.webkit.org/258888@main> Reviewed commits have been landed. Closing PR #8478 and removing active labels.