Bug 1335894 - Disable test_popupanchor.xul for linux. r=jmaher

Disable toolkit/content/tests/widgets/test_popupanchor.xul on linux machines as it permanently fais when running with Ubuntu 16.04.

MozReview-Commit-ID: HSX6xC8dG8k

--HG--
extra : rebase_source : c270b3456c92da2b49daa30cf44308b6880f5ea9

toolkit/content/tests/widgets/chrome.ini

@@ -14,7 +14,7 @@ skip-if = os == 'linux' # Bug 1115088
 [test_menubar.xul]
 skip-if = os == 'mac'
 [test_popupanchor.xul]
-skip-if = os == 'android'
+skip-if = os == 'android' || os == 'linux' # Bug 1335894 perma-fail on linux 16.04
 [test_popupreflows.xul]
 [test_tree_column_reorder.xul]
 [test_videocontrols_onclickplay.html]