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

Review URL: https://codereview.chromium.org/1212193003

Cr-Commit-Position: refs/heads/master@{#336710}


CrOS-Libchrome-Original-Commit: 547a48bb006e275f1192c983a634e911c8c1edbd
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index b1b219a..1b6d407 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -415,7 +415,7 @@
   // thread the message loop runs on, before calling Run().
   // Before BindToCurrentThread() is called only Post*Task() functions can
   // be called on the message loop.
-  scoped_ptr<MessageLoop> CreateUnbound(
+  static scoped_ptr<MessageLoop> CreateUnbound(
       Type type,
       MessagePumpFactoryCallback pump_factory);