Use StringParsingBuffer in HTMLFastPathParser. It makes the code clearer and look safer.
Pull request: https://github.com/WebKit/WebKit/pull/10209
Committed 260417@main (7b649bcd8947): <https://commits.webkit.org/260417@main> Reviewed commits have been landed. Closing PR #10209 and removing active labels.
<rdar://problem/105578836>