[net] Add AsyncDns field trial.

This adds AsyncDns field trial which sets --enable-async-dns for 50% of users
who have not configured any HostResolverImpl-related flags.

BUG=143454

Review URL: https://chromiumcodereview.appspot.com/10828373

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@152876 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed