Gardening: virtual/threaded/animations/missing-values-first-keyframe.html flaky

BUG=248938
TEST=
TBR=zmo@chromium.org

Review URL: https://codereview.chromium.org/203593006

git-svn-id: svn://svn.chromium.org/blink/trunk@169488 bbb929c8-8fbe-4397-9dbb-9b2b20218538