Add an internal flag to disable the HTML Embed Element and do so when in Lockdown Mode.
<rdar://101187413>
Pull request: https://github.com/WebKit/WebKit/pull/7788
Committed 258143@main (97790d523ce6): <https://commits.webkit.org/258143@main> Reviewed commits have been landed. Closing PR #7788 and removing active labels.