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Make AppShell always the "unknown" channel, instead of always being "stable",
because (a) it doesn't have channels, (b) we want to test unstable APIs too, and
(c) saying app_shell is "dev channel" would be insulting to chrome dev channel.

This allows APIs like chrome.hid.getUserSelectedDevices to exist in app_shell. (That particular one crashes, but what can you do...)

BUG=none
R=steel@chromium.org

Review-Url: https://codereview.chromium.org/2580063003
Cr-Commit-Position: refs/heads/master@{#442748}