Allow transitions on background-size with positionally matched keywords

This change allows CSS transitions to interpolate between lists with
matching keywords.
This affects interpolation for the following properties:
 - border-image-width
   This property is not animatable according to spec:

 - background-size
   Values are now interpolable if their keywords are positionally matched:

 - -webkit-background-size
 - -webkit-mask-size
   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%"


Cr-Commit-Position: refs/heads/master@{#397661}
9 files changed