Clean-up proxy_service_unittest.cc, to remove dependency on SingleThreadedProxyResolver.
Clean-up proxy_service_unittest.cc, to remove dependency on SingleThreadedProxyResolver.

This was a TODO generated in <http://codereview.chromium.org/149525>

This simplifies the test code since everything is running on one thread.

BUG=http://crbug.com/11746, http://crbug.com/11079

Review URL: http://codereview.chromium.org/160155

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@21829 0039d316-1c4b-4281-b951-d872f2087c98

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