Bug 1060191 - Web compat issues with window.open

https://bugzilla.mozilla.org/show_bug.cgi?id=1060191
We should not do it right here. Redirect the event to appWindowFactory to let it instantiate the AppWindow.
See https://github.com/mozilla-b2g/gaia/blob/master/apps/system/js/app_window_factory.js#L52 for example.

Note: we don't need to check the url of this window is already opened or not in AWF.