This reverts 6d25bf3aac
Invalidating the style here means that transitions can cause an element
to leave style computation with its "needs style update" flag set to
true. This then causes a VERIFY to fail in the TreeBuilder.
This invalidation does not otherwise seem to have any effect. The
original commit suggests this was to fix a bug, but it's not clear what
bug that was. If it reappears, we can try to solve the issue in a
different way.
