Web Animations CSS: Support animation of transform-origin

The added interpolation test detects some possible floating point errors in the existing implementation.

BUG=257591

Review URL: https://chromiumcodereview.appspot.com/23431025

git-svn-id: svn://svn.chromium.org/blink/trunk@158504 bbb929c8-8fbe-4397-9dbb-9b2b20218538