modalDialog: drop global.get_current_time() arguments to close

Modal dialogs take an optional timestamp in their close method.
If the timestamp is not passed in, then global.get_current_time()
is used.

Some callers of the close method pass in global.get_current_time()
unnecessarly (since it's the default).

This commit drops the argument for those cases.

https://bugzilla.gnome.org/show_bug.cgi?id=694296

js/ui/extensionDownloader.js

@@ -215,7 +215,7 @@ const InstallExtensionDialog = new Lang.Class({
     },
 
     _onCancelButtonPressed: function(button, event) {
-        this.close(global.get_current_time());
+        this.close();
         this._invocation.return_value(GLib.Variant.new('(s)', ['cancelled']));
     },
 
@@ -257,7 +257,7 @@ const InstallExtensionDialog = new Lang.Class({
             gotExtensionZipFile(session, message, uuid, dir, callback, errback);
         }));
 
-        this.close(global.get_current_time());
+        this.close();
     }
 });