Cleanups to initialValue related code.
<rdar://problem/104330630>
Pull request: https://github.com/WebKit/WebKit/pull/8720
Committed 258991@main (0e9c19d6b9a6): <https://commits.webkit.org/258991@main> Reviewed commits have been landed. Closing PR #8720 and removing active labels.
Regressed: Bug 250811 – ASSERTION FAILED: isCustomPropertyName(name) in WebCore::Style::CustomPropertyRegistry::get