Bug 877661 - (followup) - disable mask-html-01-extref-02.xhtml for frequent intermittent failures. r=me

According to bug 877661 comment 327, mark mask-html-01-extref-02.xhtml as random
before root cause of failure addressed and fixed.

MozReview-Commit-ID: 2yaIcUKjTHm

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extra : amend_source : 6174fa69a057195ed45f1078b8f1b9b913f06520