MessageLoop: wire the unused static method in base::Thread (follow-up cleanup)

MessageLoop: wire the unused static method in base::Thread (follow-up cleanup)

Actually use the private static method (CreateUnbound) in base::Thread.

I added the method in the last refactoring patch but forgot to use it;
I can either remove the static method or use it, I do the latter in this
patch as I thought leaving the private ctor of MessageLoop only for
delegated use-case cleaner.

BUG=465458
R=thakis@chromium.org

Committed: https://crrev.com/547a48bb006e275f1192c983a634e911c8c1edbd
Cr-Commit-Position: refs/heads/master@{#336710}

[kinuko, 2015-06-29 09:06:52]

Patchset #2 (id:20001) has been deleted

[kinuko, 2015-06-29 09:11:13]

Patchset #2 (id:40001) has been deleted

[kinuko, 2015-06-29 10:18:41]

Nico, I realized that my previous patch left the ML ctors in a bit incomplete
state... can you take a look at this follow-up patch?

[Nico, 2015-06-29 16:20:40]

lgtm

[kinuko, 2015-06-30 01:27:02]

The CQ bit was checked by kinuko@chromium.org

[commit-bot: I haz the power, 2015-06-30 01:27:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1212193003/60001

[commit-bot: I haz the power, 2015-06-30 01:32:03]

Committed patchset #2 (id:60001)

[commit-bot: I haz the power, 2015-06-30 01:34:02]

Patchset 2 (id:??) landed as
https://crrev.com/547a48bb006e275f1192c983a634e911c8c1edbd
Cr-Commit-Position: refs/heads/master@{#336710}