portalHelper: Set a minimum window size

Even though the window is maximised when created, the window is still
resizable. This avoids making the window smaller than usable. If the
screen is smaller than the minimum window size, the maximise should make
it fullscreen and non-resizable.

https://bugzilla.gnome.org/show_bug.cgi?id=735233

js/portalHelper/main.js

@@ -81,6 +81,7 @@ const PortalWindow = new Lang.Class({
 
         this.add(this._webView);
         this._webView.show();
+        this.set_size_request(600, 450);
         this.maximize();
         this.present_with_time(timestamp);