[chrome/browser/metrices] Replace MessageLoopProxy usage with ThreadTaskRunnerHandle

Replace MessageLoopProxy usage with ThreadTaskRunnerHandle in chrome/browser/metrices module.

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

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

MessageLoopProxy -> SingleThreadTaskRunner

BUG=391045
Committed: https://crrev.com/ddded40c2a74d1216ef43f6fba809d3ee45d9cfa
Cr-Commit-Position: refs/heads/master@{#327667}

[Pranay, 2015-04-29 06:25:41]

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

[Pranay, 2015-04-29 06:25:41]

PTAL

Thanks

[Ilya Sherman, 2015-04-30 05:07:35]

LGTM

[Pranay, 2015-04-30 05:08:01]

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

[commit-bot: I haz the power, 2015-04-30 05:08:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1114823002/1

[Pranay, 2015-04-30 05:08:23]

Thanks :)

[commit-bot: I haz the power, 2015-04-30 07:05:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-30 07:06:56]

Patchset 1 (id:??) landed as
https://crrev.com/ddded40c2a74d1216ef43f6fba809d3ee45d9cfa
Cr-Commit-Position: refs/heads/master@{#327667}