headless: Add support for host resolver mapping

headless/public/headless_browser.cc

Index: headless/public/headless_browser.cc
diff --git a/headless/public/headless_browser.cc b/headless/public/headless_browser.cc
index b66efa40724107253ab5cd2320cf55c5b5bc3ab5..194eeac5212704cfbdb851b134348f954d10f527 100644
--- a/headless/public/headless_browser.cc
+++ b/headless/public/headless_browser.cc
@@ -52,6 +52,11 @@ Builder& Builder::SetProxyServer(const net::HostPortPair& proxy_server) {
   return *this;
 }
 
+Builder& Builder::SetHostResolverRules(const std::string& host_resolver_rules) {
+  options_.host_resolver_rules = host_resolver_rules;
+  return *this;
+}
+
 Options Builder::Build() {
   return options_;
 }