mash: Shelf items outlive their windows.
On ToT @ #437019 having built 'chrome' and 'mash:all':
(1) Run chrome --mash
(2) Observe the shelf items; there should be "QuickLaunch" "Chromium - New Tab", etc.
(3) Close either window.
Expected: The corresponding shelf item disappears when the window is closed.
Actual: The corresponding shelf item is still present.