Add unit-tests for SingleRequestHostResolver.

To facilitate this, moved it out of host_resolver.cc to its own set of files.

BUG=84261
Review URL: http://codereview.chromium.org/6993015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88006 0039d316-1c4b-4281-b951-d872f2087c98