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- Quitting an app shim will close all associated windows.
- Closing all windows will quit the app shim.
- Can focus between Chrome and app shim windows.
- Can hide/show all windows associated with an app shim.

BUG=440651

Review URL: https://codereview.chromium.org/790043002

Cr-Commit-Position: refs/heads/master@{#308473}