Web Animations CSS: New virtual/threaded/animations timeout

virtual/threaded/animations/negative-delay.html is timing out on
Win Debug.

This started between r162737 and r162752, so
r162743 (http://crrev.com/83433009) is a possible cause.

TBR=dstockwell
BUG=322272

Review URL: https://codereview.chromium.org/102393003

git-svn-id: svn://svn.chromium.org/blink/trunk@163101 bbb929c8-8fbe-4397-9dbb-9b2b20218538