Rebaseline: Mark virtual/threaded/animations/missing-keyframe-properties-repeating.html flaky

BUG=350829
TBR=dstockwell

Review URL: https://codereview.chromium.org/192583002

git-svn-id: svn://svn.chromium.org/blink/trunk@168821 bbb929c8-8fbe-4397-9dbb-9b2b20218538