Web Animations CSS: Support animation of object-position

object-position controls the position of an object when using object-fit. See
the following two pages for demos of this property;
  http://dev.opera.com/articles/view/css3-object-fit-object-position/
  http://devfiles.myopera.com/articles/4682/object-fit-none-transitions.html

BUG=257591

Review URL: https://codereview.chromium.org/26805003

git-svn-id: svn://svn.chromium.org/blink/trunk@160068 bbb929c8-8fbe-4397-9dbb-9b2b20218538