Bug 1685424 - Enable test_ext_contentscript_activeTab test with fission r=robwu

Differential Revision: https://phabricator.services.mozilla.com/D110332

toolkit/components/extensions/test/mochitest/mochitest-common.ini

@@ -94,7 +94,6 @@ skip-if = headless # Bug 1405872
 [test_ext_contentscript_about_blank.html]
 skip-if = os == 'android' # bug 1369440
 [test_ext_contentscript_activeTab.html]
-skip-if = fission
 [test_ext_contentscript_cache.html]
 skip-if = (os == 'linux' && debug) || (toolkit == 'android' && debug) # bug 1348241
 fail-if = xorigin # TypeError: can't access property "staticScripts", ext is undefined - Should not throw any errors