Replace base::MessageLoop(TYPE_IO) with MessageLoopForIO.

This is the remaining clean up of testing code.

BUG=None
TEST=Ran content_unittests

Review URL: https://codereview.chromium.org/24981003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225942 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225942