[content/browser/service_worker] Using ThreadTaskRunnerHandle in lieu of MessageLoopProxy

Using ThreadTaskRunnerHandle in lieu of MessageLoopProxy  in content/browser/service_worker module

Replace MessageLoopProxy usage with ThreadTaskRunnerHandle in content/browser/service_worker module.

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

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

MessageLoopProxy -> SingleThreadTaskRunner

BUG=391045
Committed: https://crrev.com/42756a855e5e4fd4c5aa3b1e8a18fb38b7efc692
Cr-Commit-Position: refs/heads/master@{#328709}

[anujsharma, 2015-05-06 09:26:20]

anujk.sharma@samsung.com changed reviewers:
+ michaeln@chromium.org

[anujsharma, 2015-05-06 09:26:21]

PTAL

[michaeln, 2015-05-06 18:49:17]

lgtm

[anujsharma, 2015-05-07 03:17:23]

On 2015/05/06 18:49:17, michaeln wrote:
> lgtm

Thanks michaeln for lgtm.

[anujsharma, 2015-05-07 03:19:06]

The CQ bit was checked by anujk.sharma@samsung.com

[commit-bot: I haz the power, 2015-05-07 03:21:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1129753002/1

[commit-bot: I haz the power, 2015-05-07 04:50:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-07 04:51:39]

Patchset 1 (id:??) landed as
https://crrev.com/42756a855e5e4fd4c5aa3b1e8a18fb38b7efc692
Cr-Commit-Position: refs/heads/master@{#328709}