Port WTF::CString to the new IPC serialization format.
Pull request: https://github.com/WebKit/WebKit/pull/23425
Committed 273691@main (26d7b2b161f2): <https://commits.webkit.org/273691@main> Reviewed commits have been landed. Closing PR #23425 and removing active labels.
<rdar://problem/121883428>