Bug 1599984 - re-enable test_bug549170 test for ubuntu1804 r=jmaher

Changes:

Re-enable the test which now passes in ubuntu1804.

Differential Revision: https://phabricator.services.mozilla.com/D58384

--HG--
extra : moz-landing-system : lando

layout/forms/test/mochitest.ini

@@ -23,7 +23,6 @@ skip-if = toolkit == 'android' || e10s || os == 'mac' # mac(select form control
 [test_bug534785.html]
 [test_bug542914.html]
 [test_bug549170.html]
-fail-if = (os == 'linux' && os_version == '18.04') # Bug 1599984
 [test_bug562447.html]
 [test_bug563642.html]
 [test_bug564115.html]