Fix ModelPrefUpdater destruction order.

Fix ModelPrefUpdater destruction order.

This CL fixes an issue where AppListSyncableService would free the model
before the ModelPrefUpdater that was observing it.

BUG=423124
Committed: https://crrev.com/01686d7054d85bcf5d43fa5693f4c89727173fed
Cr-Commit-Position: refs/heads/master@{#300072}

[calamity, 2014-10-17 04:38:30]

calamity@chromium.org changed reviewers:
+ xiyuan@chromium.org

[calamity, 2014-10-17 04:38:30]

Thanks for diagnosing.

[xiyuan, 2014-10-17 04:58:28]

LGTM

Thank you for the quick fix.

[calamity, 2014-10-17 05:25:38]

The CQ bit was checked by calamity@chromium.org

[commit-bot: I haz the power, 2014-10-17 05:26:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/661913002/1

[commit-bot: I haz the power, 2014-10-17 06:30:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-17 06:30:54]

Patchset 1 (id:??) landed as
https://crrev.com/01686d7054d85bcf5d43fa5693f4c89727173fed
Cr-Commit-Position: refs/heads/master@{#300072}