Web Animations CSS: Fix repeated-firing-background-color.html

The Web Animations engine correctly fires end events before the animation has
terminated, causing this test to fail unexpectedly. This change relaxes the
test to avoid the failure, while still enforcing the original intent of the
test.

R=timloh@chromium.org
BUG=258896

Review URL: https://codereview.chromium.org/74783009

git-svn-id: svn://svn.chromium.org/blink/trunk@162267 bbb929c8-8fbe-4397-9dbb-9b2b20218538