Add user-agent flag to headless

headless/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/chrome_build.gni")
 import("//headless/headless.gni")
 import("//build/util/process_version.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//testing/test.gni")
 import("//tools/grit/grit_rule.gni")
@@ skipping 172 matching lines @@
   args = [
     "--protocol",
     rebase_path(inputs[0], root_build_dir),
     "--output_dir",
     rebase_path(target_gen_dir) + "/public",
   ]
 }
 
 static_library("headless_lib") {
   sources = generated_devtools_api + [
+              "app/headless_shell_switches.cc",
+              "app/headless_shell_switches.h",
               "lib/browser/headless_browser_context_impl.cc",
               "lib/browser/headless_browser_context_impl.h",
               "lib/browser/headless_browser_context_options.cc",
               "lib/browser/headless_browser_context_options.h",
               "lib/browser/headless_browser_impl.cc",
               "lib/browser/headless_browser_impl.h",
               "lib/browser/headless_browser_impl_mac.mm",
               "lib/browser/headless_browser_main_parts_mac.mm",
               "lib/browser/headless_browser_main_parts.cc",
               "lib/browser/headless_browser_main_parts.h",
@@ skipping 281 matching lines @@
 
 executable("headless_example") {
   sources = [
     "app/headless_example.cc",
   ]
 
   deps = [
     "//headless:headless_shell_lib",
   ]
 }