Mark test as flaky crash

This issue seems to be related to the new Web Animations engine.

TBR=dstockwell@chromium.org
BUG=343185

Review URL: https://codereview.chromium.org/137493010

git-svn-id: svn://svn.chromium.org/blink/trunk@167028 bbb929c8-8fbe-4397-9dbb-9b2b20218538