Make net::HostResolver refcounted.

This way it can be properly shared between the url request contexts, and the dns prefetcher, and dns observer.

BUG=http://crbug.com/14664
TEST=existing unit tests.

Review URL: http://codereview.chromium.org/149053

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@19451 0039d316-1c4b-4281-b951-d872f2087c98
28 files changed