[chrome/browser] Replace MessageLoopProxy usage with ThreadTaskRunnerHandle

Replace MessageLoopProxy usage with ThreadTaskRunnerHandle in chrome/browser module.

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

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

MessageLoopProxy -> SingleThreadTaskRunner

BUG=391045
Committed: https://crrev.com/6a179254d7d55b3987eb4d1a2c64aa5ad1de5d76
Cr-Commit-Position: refs/heads/master@{#328310}

[Pranay, 2015-05-04 07:08:05]

pranay.kumar@samsung.com changed reviewers:
+ bauerb@chromium.org, blundell@chromium.org, kalman@chromium.org,
peter@chromium.org, sky@chromium.org

[Pranay, 2015-05-04 07:08:05]

kalman@chromium.org: Please review changes in chrome/browser/apps

peter@chromium.org: Please review changes in chrome/browser/notifications


blundell@chromium.org: Please review changes in chrome/browser/signin


bauerb@chromium.org: Please review changes in chrome/browser/supervised_user &
chrome/browser/prefs

sky@chromium.org: Please review changes in chrome/browser

[blundell, 2015-05-04 08:46:26]

blundell@chromium.org changed reviewers:
+ rogerta@chromium.org

[blundell, 2015-05-04 08:46:27]

blundell -> rogerta

[not at google - send to devlin, 2015-05-04 13:53:41]

apps lgtm

[sky, 2015-05-04 15:29:48]

LGTM

[Pranay, 2015-05-05 05:30:04]

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

[commit-bot: I haz the power, 2015-05-05 05:30:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1112403005/20001

[Pranay, 2015-05-05 05:48:52]

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

[Pranay, 2015-05-05 05:50:53]

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

[commit-bot: I haz the power, 2015-05-05 05:51:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1112403005/20001

[Pranay, 2015-05-05 05:54:35]

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

[Bernhard Bauer, 2015-05-05 07:16:15]

lgtm

[Pranay, 2015-05-05 07:43:05]

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

[Pranay, 2015-05-05 07:43:22]

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

[Pranay, 2015-05-05 07:43:24]

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

[Pranay, 2015-05-05 07:43:44]

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

[commit-bot: I haz the power, 2015-05-05 07:43:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1112403005/20001

[Pranay, 2015-05-05 11:31:26]

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

[commit-bot: I haz the power, 2015-05-05 11:32:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1112403005/20001

[commit-bot: I haz the power, 2015-05-05 11:35:35]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-05-05 11:36:27]

Patchset 2 (id:??) landed as
https://crrev.com/6a179254d7d55b3987eb4d1a2c64aa5ad1de5d76
Cr-Commit-Position: refs/heads/master@{#328310}