Show the modal overlay as soon as openWindow is called

Dialogs can take a while to load, especially with iframes, so
show the overlay as soon as open is called so the user gets
instant visual feedback that their action registered.

Bug: 68669
Change-Id: I8046a454657a7e40dc0c5f07304b3da5d42a1001
Merge "Show the modal overlay as soon as openWindow is called"