Port HTTPHeaderNames to the new serialization format
<rdar://problem/105970730>
Pull request: https://github.com/WebKit/WebKit/pull/10723
Pull request: https://github.com/WebKit/WebKit/pull/15949
Committed 266737@main (c553523612a5): <https://commits.webkit.org/266737@main> Reviewed commits have been landed. Closing PR #15949 and removing active labels.