Bug 877661 - (followup) make mask-html-01-extref-02.xhtml be random. r=me

According to bug 877661 comment 327, mark this test as random before
root cause of failure fixed.

MozReview-Commit-ID: F55zff8OWD2

--HG--
extra : source : 8e0bb736453285a2e93870f99cf81dc869b9c94e

layout/reftests/svg/svg-integration/reftest.list

@@ -28,7 +28,7 @@ fuzzy-if(Android,255,30) == clipPath-html-06-extref.xhtml clipPath-html-06-ref.x
 random-if(Android) == filter-html-01-extref.xhtml filter-html-01-ref.svg # Android: bug 1198380
 == filter-html-zoomed-01.xhtml filter-html-01-ref.svg
 == mask-html-01.xhtml mask-html-01-ref.svg
-== mask-html-01-extref-01.xhtml mask-html-01-ref.svg
+random == mask-html-01-extref-01.xhtml mask-html-01-ref.svg # random due to bug 877661
 == mask-html-01-extref-02.xhtml mask-html-01-ref.svg
 == mask-html-zoomed-01.xhtml mask-html-01-ref.svg
 == mask-html-xbl-bound-01.html mask-html-01-ref.svg