Gardening: mark virtual/threaded/animations/duplicated-keyframes-name-unprefixed-03.html as flaky

BUG=350829
TBR=esprehn

Review URL: https://codereview.chromium.org/297343004

git-svn-id: svn://svn.chromium.org/blink/trunk@174798 bbb929c8-8fbe-4397-9dbb-9b2b20218538