Web Animations: Compositor timeOffset should not be scaled

This also seems to indicate that it's not yet possible to composite an
animation with a negative playback rate and infinite duration.

BUG=422432

Review URL: https://codereview.chromium.org/667633003

git-svn-id: svn://svn.chromium.org/blink/trunk@183957 bbb929c8-8fbe-4397-9dbb-9b2b20218538