Fix the layer hosting added in 269029@main to make plugins more similar to iframes.
<rdar://problem/116635683>
Pull request: https://github.com/WebKit/WebKit/pull/18821
Committed 269063@main (6f0db138bd4a): <https://commits.webkit.org/269063@main> Reviewed commits have been landed. Closing PR #18821 and removing active labels.