Mark layout test animation-direction-reverse-fill-mode.html also as timeout

Layout test virtual/web-animations-css/animations/animation-direction-reverse-fill-mode.html also times out sometimes.

BUG=258896
TEST=none
TBR=dominicc@chromium.org

Review URL: https://codereview.chromium.org/23112012

git-svn-id: svn://svn.chromium.org/blink/trunk@156508 bbb929c8-8fbe-4397-9dbb-9b2b20218538