Add HeadlessFocusClient to fix document.hasFocus() issues.

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 249 matching lines @@
               "public/util/url_fetcher.cc",
               "public/util/url_fetcher.h",
               "public/util/url_request_dispatcher.h",
               "public/util/user_agent.cc",
               "public/util/user_agent.h",
             ]
 
   if (use_aura) {
     sources += [
       "lib/browser/headless_browser_impl_aura.cc",
+      "lib/browser/headless_focus_client.cc",
+      "lib/browser/headless_focus_client.h",
       "lib/browser/headless_screen.cc",
       "lib/browser/headless_screen.h",
       "lib/browser/headless_window_parenting_client.cc",
       "lib/browser/headless_window_parenting_client.h",
       "lib/browser/headless_window_tree_host.cc",
       "lib/browser/headless_window_tree_host.h",
     ]
   }
 
   deps = [
@@ skipping 191 matching lines @@
 
 executable("headless_example") {
   sources = [
     "app/headless_example.cc",
   ]
 
   deps = [
     "//headless:headless_shell_lib",
   ]
 }