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/178263006

git-svn-id: svn://svn.chromium.org/blink/trunk@170491 bbb929c8-8fbe-4397-9dbb-9b2b20218538