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 127c97b2360221e051d21e4fffb51a43295fdd71..cd4902345f331de03a817e948e58b00018f5c536 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)));