[Extensions UI Mac] Move popup close logic to ExtensionPopupController
Move the logic to close an extension popup to the ExtensionPopupController.
In addition to being a little cleaner design, this allows us to remove some of
the accessors on the ExtensionPopupController.
This also should fix crbug.com/484538, which I suspect is happening when we
try and close the popup after the extension is removed.
BUG=484538
Review URL: https://codereview.chromium.org/
1126823002
Cr-Commit-Position: refs/heads/master@{#328449}