Fix to ensure IO thread is created when ExtensionUpdater needs it.
Fix to ensure IO thread is created when ExtensionUpdater needs it.

In some cases on MacOS it turned out that the IO Thread hadn't been
created at the time we were asking for it.

BUG=http://crbug.com/20794
TEST=none

Review URL: http://codereview.chromium.org/184024

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@25277 0039d316-1c4b-4281-b951-d872f2087c98

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