Web Animations: interpolate specified length values rather than computed length values.

Note that this change only applies to animations generated via the API. CSS transitions and animations are not impacted by this patch.

This allows animations to remain responsive to changes in computed style, including when in fill. So, for example:

foo.animate({width: '30em'}, {duration: 1, fill: 'forwards'});

will animate to 30em and then stay at 30em (i.e. responsive to changes in font size).

Review URL: https://codereview.chromium.org/230253002

git-svn-id: svn://svn.chromium.org/blink/trunk@171195 bbb929c8-8fbe-4397-9dbb-9b2b20218538