Revert of [mojo] Replace MessageLoopProxy usage with ThreadTaskRunnerHandle

Revert of [mojo] Replace MessageLoopProxy usage with ThreadTaskRunnerHandle (patchset #3 id:40001 of https://codereview.chromium.org/1118473002/)

Reason for revert:
breaks compilation on win gn
https://build.chromium.org/p/chromium.win/builders/Win8%20GN/builds/6922

Original issue's description:
> Replace MessageLoopProxy usage with ThreadTaskRunnerHandle in mojo module.
>
> MessageLoopProxy is deprecated.
> This basically does a search and replace:
>
> MessageLoopProxy::current() -> ThreadTaskRunnerHandle::Get().
>
> BUG=391045
>
> Committed: https://crrev.com/332a1b2b1d5635199d5ed453f5f9586b2e030b64
> Cr-Commit-Position: refs/heads/master@{#328311}

TBR=sky@chromium.org,anujk.sharma@samsung.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=391045
Committed: https://crrev.com/b77bd75920b697f0c1b06e74b760d308bb863e42
Cr-Commit-Position: refs/heads/master@{#328314}

[jochen (gone - plz use gerrit), 2015-05-05 12:15:09]

Created Revert of [mojo] Replace MessageLoopProxy usage with
ThreadTaskRunnerHandle

[commit-bot: I haz the power, 2015-05-05 12:15:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1125913002/1

[commit-bot: I haz the power, 2015-05-05 12:16:06]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-05 12:17:05]

Patchset 1 (id:??) landed as
https://crrev.com/b77bd75920b697f0c1b06e74b760d308bb863e42
Cr-Commit-Position: refs/heads/master@{#328314}