Allow transitions on background-size with positionally matched keywords

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:
   https://drafts.csswg.org/css-backgrounds-3/#the-border-image-width

 - background-size
   Values are now interpolable if their keywords are positionally matched:
   https://github.com/w3c/csswg-drafts/commit/a5f9de921fa7834c79ee15660d9d86528bce1607

 - -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%"

BUG=616072
Review-Url: https://codereview.chromium.org/2027933002
Cr-Commit-Position: refs/heads/master@{#397661}
(cherry picked from commit 8793a7bac4d2d56e38c8bd406920128e1ceb7a9a)

[alancutter (OOO until 2018), 2016-06-07 07:23:37]

Committed patchset #1 (id:1) to pending queue manually as
f0477aee496855d07d041c6a3553eb496d6c5655.