[components/policy] Use ThreadTaskRunnerHandle instead of LoopProxy

Replace MessageLoopProxy usage with ThreadTaskRunnerHandle in components/policy module.

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

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

BUG=391045
Committed: https://crrev.com/8956bcd3f54895359c610eccf9b5e87626035f53
Cr-Commit-Position: refs/heads/master@{#329386}

[Pranay, 2015-05-06 12:29:23]

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

[Pranay, 2015-05-06 12:29:24]

Hi Erik,

PTAL, 
Thanks

[erikwright (departed), 2015-05-11 18:30:06]

erikwright@chromium.org changed reviewers:
- erikwright@chromium.org

[erikwright (departed), 2015-05-11 18:30:06]

Please select an owner from
https://code.google.com/p/chromium/codesearch#chromium/src/components/policy/...

[Pranay, 2015-05-12 02:48:17]

On 2015/05/11 18:30:06, erikwright wrote:
> Please select an owner from
>
https://code.google.com/p/chromium/codesearch#chromium/src/components/policy/...

Sure Erik :) thanks

[Pranay, 2015-05-12 02:48:40]

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

[Pranay, 2015-05-12 02:48:41]

PTAL

Thanks

[bartfab (slow), 2015-05-12 08:36:04]

LGTM

[Pranay, 2015-05-12 09:41:19]

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

[commit-bot: I haz the power, 2015-05-12 09:41:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1130753002/1

[commit-bot: I haz the power, 2015-05-12 10:19:58]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-12 10:21:06]

Patchset 1 (id:??) landed as
https://crrev.com/8956bcd3f54895359c610eccf9b5e87626035f53
Cr-Commit-Position: refs/heads/master@{#329386}