Port 1552538 - Check for gBrowser before checking for pinned tabs r=r1cky

lib/ASRouterTargeting.jsm

@@ -298,7 +298,7 @@ const TargetingGetters = {
   },
   get hasPinnedTabs() {
     for (let win of Services.wm.getEnumerator("navigator:browser")) {
-      if (win.closed) {
+      if (win.closed || !win.ownerGlobal.gBrowser) {
         continue;
       }
       if (win.ownerGlobal.gBrowser.visibleTabs.filter(t => t.pinned).length) {