[net] Use only DnsConfigService to detect DNS changes in HostResolverImpl

The signals coming currently via DNSObserver are too noisy. DnsConfigService
can weed out false positives, so switch to using it exclusively.

BUG=142138
TEST=network requests work after network changes


Review URL: https://chromiumcodereview.appspot.com/10830271

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@151278 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed