Bug 1494023 Add missing head.js for extension browser tests r=zombie

Differential Revision: https://phabricator.services.mozilla.com/D6816

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

browser/components/extensions/test/browser/browser.ini

@@ -1,6 +1,9 @@
 [DEFAULT]
 tags = webextensions in-process-webextensions
 
+support-files =
+  head.js
+
 [browser_ext_autocompletepopup.js]
 [browser_ext_legacy_extension_context_contentscript.js]
 [browser_ext_windows_allowScriptsToClose.js]