HashMapStylePropertyMap is readonly and shouldn't be subclassing StylePropertyMap. Instead, introduce a MainThreadStylePropertyMapReadonly and have both HashMapStylePropertyMap and StylePropertyMap subclass it.
Pull request: https://github.com/WebKit/WebKit/pull/5765
Committed 256012@main (b902c3b0e0c7): <https://commits.webkit.org/256012@main> Reviewed commits have been landed. Closing PR #5765 and removing active labels.
<rdar://problem/101588833>