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}