[BackgroundSync] Fix lifetime issues with callbacks pointer in CallbackThreadAdapter

[BackgroundSync] Fix lifetime issues with callbacks pointer in CallbackThreadAdapter

This correctly assigns ownership of the callbacks_ object to the closure created
by base::Bind when posting the success/error handling task to the correct
thread.

BUG=488452
Committed: https://crrev.com/80341c95839b9eda74398825d63c91f22a5a3eec
Cr-Commit-Position: refs/heads/master@{#330094}

[iclelland, 2015-05-15 12:56:00]

iclelland@chromium.org changed reviewers:
+ jkarlin@chromium.org

[jkarlin, 2015-05-15 13:03:04]

The CQ bit was checked by jkarlin@chromium.org

[jkarlin, 2015-05-15 13:03:04]

lgtm

[commit-bot: I haz the power, 2015-05-15 13:03:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1139643008/1

[commit-bot: I haz the power, 2015-05-15 14:40:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-15 14:41:13]

Patchset 1 (id:??) landed as
https://crrev.com/80341c95839b9eda74398825d63c91f22a5a3eec
Cr-Commit-Position: refs/heads/master@{#330094}