[chrome/chrome_watcher] Replace MessageLoopProxy usage with ThreadTaskRunnerHandle

Replace MessageLoopProxy usage with ThreadTaskRunnerHandle in chrome watcher module.

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

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

BUG=391045
Committed: https://crrev.com/c8c0ede792816b36fb0bfae90549190794748ffd
Cr-Commit-Position: refs/heads/master@{#327363}

[Pranay, 2015-04-28 05:33:11]

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

[Pranay, 2015-04-28 05:33:12]

PTAL

[erikwright (departed), 2015-04-28 17:26:27]

LGTM

[Pranay, 2015-04-28 18:50:42]

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

[commit-bot: I haz the power, 2015-04-28 18:51:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1103343003/1

[commit-bot: I haz the power, 2015-04-28 21:00:25]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-28 21:01:18]

Patchset 1 (id:??) landed as
https://crrev.com/c8c0ede792816b36fb0bfae90549190794748ffd
Cr-Commit-Position: refs/heads/master@{#327363}