[net] Asynchronous DNS ready for experiments.

If started with --enable-async-dns, HostResolverImpl will use
DnsConfigService to determine system DNS configuration and
DnsTransaction to resolve host names. It will fallback to
HostResolverProc on failure.

BUG=90881, 107880, 113829
TEST=./net_unittests --gtest_filter=HostResolverImpl*:Dns*


Review URL: http://codereview.chromium.org/9369045

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@122878 0039d316-1c4b-4281-b951-d872f2087c98
31 files changed