Mark virtual/web-animations-css/animations/play-state-initially-paused.html as flaky

BUG=321841
TEST=
TBR=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/77423003

git-svn-id: svn://svn.chromium.org/blink/trunk@162422 bbb929c8-8fbe-4397-9dbb-9b2b20218538