CSS Animations: Fix issues related to interpolation of -webkit-filter and add support in Web Animations

* Cleans up the pass-through logic for interpolation with 'none'.
* Removes the pass-through filter as it is no longer needed.
* Adds support for interpolating between filter lists of different
  lengths (as described in editors draft). Previously this only
  happened when one list was empty.
* Fall back to step interpolation when either filter list specifies a
  reference (as described in editors draft).
* Clamps filter values (as described in editors draft).

The differences between the existing implementation and web-animations
are related to default interpolation stepping at 0% vs 50% and web-animations
using specified values at 0 and 1. The remaining failures under 
web-animations are due to the drop-shadow filter using transparent
instead of currentcolor.

Spec: https://dvcs.w3.org/hg/FXTF/raw-file/default/filters/index.html#interpolation-of-filters

BUG=257591,305065,305067

Review URL: https://codereview.chromium.org/26298006

git-svn-id: svn://svn.chromium.org/blink/trunk@160522 bbb929c8-8fbe-4397-9dbb-9b2b20218538