Fix for DCHECK failure at startup of gin_shell.

Fix for DCHECK failure at startup of gin_shell.

It was introduced by rewriting code to use SingeThreadTaskRunner
instead of MessageLoopProxy.

Reproduces with gin_unittests too.

BUG=

Committed: https://crrev.com/65e946531a3de45739107ea1e2a58a314d245f9b
Cr-Commit-Position: refs/heads/master@{#332597}

[Krzysztof Olczyk, 2015-06-03 10:01:14]

kolczyk@opera.com changed reviewers:
+ jochen@chromium.org

[Krzysztof Olczyk, 2015-06-03 10:01:15]

Hi jochen,

could you review this small fix for failing DCHECKs in gin_shell and
gin_unittests?

[jochen (gone - plz use gerrit), 2015-06-03 11:18:26]

lgtm

[Krzysztof Olczyk, 2015-06-03 11:22:52]

The CQ bit was checked by kolczyk@opera.com

[commit-bot: I haz the power, 2015-06-03 11:23:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1153473009/1

[commit-bot: I haz the power, 2015-06-03 11:50:01]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-03 11:51:08]

Patchset 1 (id:??) landed as
https://crrev.com/65e946531a3de45739107ea1e2a58a314d245f9b
Cr-Commit-Position: refs/heads/master@{#332597}