When a CSS animation keyframe uses var() referencing a nonexistent or
invalid custom property, variable substitution produces a
guaranteed-invalid value. The animation keyframe processing code did not
handle this case, allowing the value to reach compute_opacity() (and
similar functions) which would hit VERIFY_NOT_REACHED().
Fix this by skipping guaranteed-invalid values in
compute_keyframe_values, matching how the regular cascading code treats
them.
This fixes a crash on chess.com.
