OverlayWindow will allow apps to place an iframe on top of the front-most app. It won't overlay any system UI. It will be closed after a home button click. It won't allow passing clicks to the overlaid content (as the GlobalOverlayWindow does) and it will have the background transparent but grayed out. OverlayWindow instances will be created via |window.open(..., ..., 'overlay')| and we want this to be exposed to any web content.