According to bug 877661 comment 327, mark mask-html-01-extref-02.xhtml as random before
root cause of failure addressed and fixed.
In previous followup, I put random on wrong test case. This patch correct the
error of previous commit(9a8e1601b493).

MozReview-Commit-ID: 7Q7TI9TtgRW

--HG--
extra : amend_source : fb1407e8ca5b1f8015fc3d85d3e0c6cdc4d1a33b