Bug 1620364 - Always register for :CloseTab. r=droeh

Mistakenly removed these lines which are still needed until we remove the
deprecated API.

Differential Revision: https://phabricator.services.mozilla.com/D65594

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

mobile/android/geckoview/src/main/java/org/mozilla/geckoview/WebExtension.java

@@ -816,6 +816,11 @@ public class WebExtension {
 
         public Listener(final GeckoSession session) {
             this(session, null);
+            // TODO: remove Bug 1618987
+            // Close tab event is forwarded to the main listener so we need to listen
+            // to it here.
+            mEventDispatcher.registerUiThreadListener(this,
+                    "GeckoView:WebExtension:CloseTab");
         }
 
         private Listener(final GeckoSession session, final GeckoRuntime runtime) {