headless: Add support for host resolver mapping

headless/app/headless_shell.cc

Index: headless/app/headless_shell.cc
diff --git a/headless/app/headless_shell.cc b/headless/app/headless_shell.cc
index c17c9efca1b091f5a29393df80e72d2bf43f2d3e..e1348e328ed10634668affd59a479af1b29429ed 100644
--- a/headless/app/headless_shell.cc
+++ b/headless/app/headless_shell.cc
@@ -127,6 +127,11 @@ int main(int argc, const char** argv) {
     builder.SetProxyServer(parsed_proxy_server);
   }
 
+  if (command_line.HasSwitch(switches::kHostResolverRules)) {
+    builder.SetHostResolverRules(
+        command_line.GetSwitchValueASCII(switches::kHostResolverRules));
+  }
+
   return HeadlessBrowserMain(
       builder.Build(),
       base::Bind(&HeadlessShell::OnStart, base::Unretained(&shell)));