Mark use-while-animating-crash.html flaky.

BUG=337774
TBR=hclam@chromium.org

Review URL: https://codereview.chromium.org/146613002

git-svn-id: svn://svn.chromium.org/blink/trunk@165721 bbb929c8-8fbe-4397-9dbb-9b2b20218538