Fix LayoutTest animations/animation-direction-reverse-fill-mode.html

This was testing forwards fill incorrectly. The Web Animations implementation
is correct, the legacy implementation is not.

R=dstockwell@chromium.org, dstockwell

Review URL: https://codereview.chromium.org/34613014

git-svn-id: svn://svn.chromium.org/blink/trunk@160298 bbb929c8-8fbe-4397-9dbb-9b2b20218538