Allow transitions on background-size with positionally matched keywords
This change allows CSS transitions to interpolate between lists with
This affects interpolation for the following properties:
This property is not animatable according to spec:
Values are now interpolable if their keywords are positionally matched:
These mirror the animation behaviour of background-size.
This change only affects CSS Transitions for the sake of a small mergable patch.
CSS Animations behaviour will be brought in line in a separate patch.
This change ensures we have compatible behaviour with other browsers for
transitioning between single length background-size values e.g.
"background-size: 50%" to "background-size: 100%"