Address FIXMEs in process-css-properties.py leftover from the transition from perl
<rdar://problem/102292266>
Pull request: https://github.com/WebKit/WebKit/pull/6444
Committed 256763@main (b45acc1caf6b): <https://commits.webkit.org/256763@main> Reviewed commits have been landed. Closing PR #6444 and removing active labels.