Mark virtual/threaded/animations/keyframes-unprefixed-03.html as slow

TBR=jsbell@chromium.org
BUG=311482

Review URL: https://codereview.chromium.org/243103009

git-svn-id: svn://svn.chromium.org/blink/trunk@172048 bbb929c8-8fbe-4397-9dbb-9b2b20218538