Mac: Remove download shelf from AX hierarchy when not visible

Mac: Remove download shelf from AX hierarchy when not visible.

When the download shelf is closed, it's still in the view hierarchy with height
of 0. To make it not appear in the AX hierarchy, we hide the view after the
close animation is finished.

BUG=417698
Committed: https://crrev.com/d7d85e9d27acfb64033866553f9e590707a84530
Cr-Commit-Position: refs/heads/master@{#297300}

[Andre, 2014-09-29 22:17:00]

andresantoso@chromium.org changed reviewers:
+ avi@chromium.org

[Andre, 2014-09-29 22:17:01]

Avi, please review.

[Avi (use Gerrit), 2014-09-29 22:19:32]

lgtm

[Andre, 2014-09-29 22:21:31]

The CQ bit was checked by andresantoso@chromium.org

[commit-bot: I haz the power, 2014-09-29 22:22:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/615663002/20001

[commit-bot: I haz the power, 2014-09-29 23:22:41]

Committed patchset #2 (id:20001) as 12ad54eed5c69463eaad9ac01dfd91a1c9689b45

[commit-bot: I haz the power, 2014-09-29 23:23:33]

Patchset 2 (id:??) landed as
https://crrev.com/d7d85e9d27acfb64033866553f9e590707a84530
Cr-Commit-Position: refs/heads/master@{#297300}