[chrome/browser/chromeos/login] Replace MessageLoopProxy usage with ThreadTaskRunnerHandle

Replace MessageLoopProxy usage with ThreadTaskRunnerHandle in chrome/browser/chromeos/login module.

MessageLoopProxy is deprecated.
This basically does a search and replace of:

MessageLoopProxy::current() -> ThreadTaskRunnerHandle::Get().

MessageLoopProxy -> SingleThreadTaskRunner

BUG=391045
Committed: https://crrev.com/9b50d88e85bf17a978857d307783ef31adad55be
Cr-Commit-Position: refs/heads/master@{#328479}

[Pranay, 2015-05-05 06:29:08]

pranay.kumar@samsung.com changed reviewers:
+ piman@chromium.org

[Pranay, 2015-05-05 06:29:09]

PTAL

Thanks

[piman, 2015-05-05 17:54:28]

lgtm

[Pranay, 2015-05-06 02:42:10]

The CQ bit was checked by pranay.kumar@samsung.com

[commit-bot: I haz the power, 2015-05-06 02:42:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1121233010/1

[commit-bot: I haz the power, 2015-05-06 03:10:32]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-06 03:11:34]

Patchset 1 (id:??) landed as
https://crrev.com/9b50d88e85bf17a978857d307783ef31adad55be
Cr-Commit-Position: refs/heads/master@{#328479}