[Metro] Set a shelf icon for the user manager.

[Metro] Set a shelf icon for the user manager.

Otherwise, minimising it sends it into the void, from where it can
never be recovered (unless you manually re-open it again)

BUG=450959
TEST=Start Chrome with --enable-new-avatar menu, and in Metro mode.
Choose "Switch Person" from the avatar menu. The window should have a
shelf item.

Committed: https://crrev.com/48a41be3fc0c9619197d09b97e8677ea9c866a42
Cr-Commit-Position: refs/heads/master@{#314236}

[noms (inactive), 2015-02-02 23:20:28]

noms@chromium.org changed reviewers:
+ sky@chromium.org

[noms (inactive), 2015-02-02 23:20:28]

PTAL. Thanks!

The shelf icon looks like a folder -- From the description, it looks like the
most appropriate, and it's probably a slight improvement over re-using the
Chrome icon. Let me know if you think I should use a different icon, though.

[sky, 2015-02-03 00:18:38]

LGTM

[noms (inactive), 2015-02-03 00:34:19]

The CQ bit was checked by noms@chromium.org

[commit-bot: I haz the power, 2015-02-03 00:35:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/891173005/1

[commit-bot: I haz the power, 2015-02-03 01:32:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-03 01:33:44]

Patchset 1 (id:??) landed as
https://crrev.com/48a41be3fc0c9619197d09b97e8677ea9c866a42
Cr-Commit-Position: refs/heads/master@{#314236}