Handle direction control in compositor Animations

This patch is the blink side. The chromium side is handled on issue #180153010

It doesn't revert functions anymore in blink. Instead, it sets the direction parameter to WebAnimation, which will in turn pass it to the compositor.

BUG=348071

Review URL: https://codereview.chromium.org/225183014

git-svn-id: svn://svn.chromium.org/blink/trunk@181945 bbb929c8-8fbe-4397-9dbb-9b2b20218538