Web Animations API: Support non-animatable properties being animated via element.animate()

CSS Animations and Transitions have a select number of properties that
are allowed to be animated. Web Animations allows for all non-animation
properties falling back to a default 50% flip interpolation between values.
This patch adds support to element.animate() for the non-animatable
non-animation properties.

Review URL: https://codereview.chromium.org/225653003

git-svn-id: svn://svn.chromium.org/blink/trunk@173067 bbb929c8-8fbe-4397-9dbb-9b2b20218538