BuilderCustom::applyValueWillChange just converts a CSSValue to WillChangeData, this is something BuilderConverter can do as well.
<rdar://problem/112329065>
Pull request: https://github.com/WebKit/WebKit/pull/15867
Committed 266088@main (3ae98fa2c02b): <https://commits.webkit.org/266088@main> Reviewed commits have been landed. Closing PR #15867 and removing active labels.