Revert of base: Remove use of MessageLoopProxy

Revert of base: Remove use of MessageLoopProxy (patchset #6 id:100001 of https://codereview.chromium.org/1100773004/)

Reason for revert:
This CL caused this failure:
http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%281%29/builds/2126

I would strongly recommend doing this in smaller pieces since it combines mechanical changes with more subtle ones (base/prefs, base/task).

Original issue's description:
> base: Remove use of MessageLoopProxy
>
> Replace usage of MessageLoopProxy under base/ with SingleThreadTaskRunner
> and ThreadTaskRunnerHandle (excluding the implementation of MessageLoopProxy
> itself which will removed later).
>
> This patch was mostly autogenerated with
> https://codereview.chromium.org/1010073002.
>
> Depends on https://codereview.chromium.org/1086733002/.
>
> BUG=465354
> TBR=nkostylev@chromium.org,pkasting@chromium.org,pauljensen@chromium.org
>
> Committed: https://crrev.com/62aa5ca413e15738ebebbb9acd271138ec808739
> Cr-Commit-Position: refs/heads/master@{#327512}

TBR=danakj@chromium.org,skyostil@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=465354
Committed: https://crrev.com/7669182866d8605d98a5141f7b82be3b516799ca
Cr-Commit-Position: refs/heads/master@{#327573}

[stevenjb, 2015-04-29 21:17:29]

Created Revert of base: Remove use of MessageLoopProxy

[commit-bot: I haz the power, 2015-04-29 21:17:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1113953002/1

[commit-bot: I haz the power, 2015-04-29 21:43:19]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-29 21:44:45]

Patchset 1 (id:??) landed as
https://crrev.com/7669182866d8605d98a5141f7b82be3b516799ca
Cr-Commit-Position: refs/heads/master@{#327573}