Fix LayoutTest animations/fill-mode-iteration-count-non-integer.html

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

R=dstockwell

Review URL: https://codereview.chromium.org/36303005

git-svn-id: svn://svn.chromium.org/blink/trunk@160297 bbb929c8-8fbe-4397-9dbb-9b2b20218538