This is no longer needed as SourceBufferPrivate inherit from CanMakeWeakPtr
<rdar://problem/107769670>
Pull request: https://github.com/WebKit/WebKit/pull/12531
Committed 262750@main (03e0165f167e): <https://commits.webkit.org/262750@main> Reviewed commits have been landed. Closing PR #12531 and removing active labels.