Bug 1508853 make the search default panel persistent, r=aswan

This is a quick fix to ensure that the search install panel is shown when an extension uses is_default.  The intention is to uplift for 64.

Differential Revision: https://phabricator.services.mozilla.com/D13078

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

browser/modules/ExtensionsUI.jsm

@@ -365,7 +365,7 @@ var ExtensionsUI = {
       let popupOptions = {
         hideClose: true,
         popupIconURL: icon || DEFAULT_EXTENSION_ICON,
-        persistent: false,
+        persistent: true,
         removeOnDismissal: true,
         eventCallback(topic) {
           if (topic == "removed") {