Don't use raw pointers in XSLImportRule and XSLStyleSheet.
Pull request: https://github.com/WebKit/WebKit/pull/3522
Committed 253695@main (896fddc4ea9a): <https://commits.webkit.org/253695@main> Reviewed commits have been landed. Closing PR #3522 and removing active labels.
<rdar://problem/99047764>