Animation: Run fractional iteration counts on compositor

Compositor now supports fractional iteration counts and animations now
utilise this.

This depends on http://crrev.com/423373003

BUG=398767

Review URL: https://codereview.chromium.org/437453004

git-svn-id: svn://svn.chromium.org/blink/trunk@180522 bbb929c8-8fbe-4397-9dbb-9b2b20218538