Call callback asynchronously for device reboot command

Call callback asynchronously for device reboot command

The remote command framework requires the remote job implementations
to call the callback asynchronously. Post task to the message loop
of current thread to fix this issue.

BUG=480982
Committed: https://crrev.com/1777b8b7977c9dddb5f64fffb897a80ed58ebbdf
Cr-Commit-Position: refs/heads/master@{#326789}

[binjin, 2015-04-24 12:57:19]

binjin@chromium.org changed reviewers:
+ bartfab@chromium.org, cschuet@chromium.org

[binjin, 2015-04-24 12:57:54]

Please take a quick look at this one-line fix.

[bartfab (slow), 2015-04-24 13:59:38]

Please reference a bug number. Other than that, LGTM.

[binjin, 2015-04-24 14:05:45]

The CQ bit was checked by binjin@chromium.org

[commit-bot: I haz the power, 2015-04-24 14:06:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1102023002/1

[commit-bot: I haz the power, 2015-04-24 14:20:29]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-24 14:21:25]

Patchset 1 (id:??) landed as
https://crrev.com/1777b8b7977c9dddb5f64fffb897a80ed58ebbdf
Cr-Commit-Position: refs/heads/master@{#326789}