Fix LayoutTest animations/fill-mode-reverse.html

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

Also, the behavior it tests is mostly covered by
animations/animation-direction-reverse-fill-mode.html, so it should be merged
with that test.

R=dstockwell

Review URL: https://codereview.chromium.org/36543002

git-svn-id: svn://svn.chromium.org/blink/trunk@160322 bbb929c8-8fbe-4397-9dbb-9b2b20218538