Revert of [headless] Use individual aura::WindowTreeHosts per WebContents.

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 203 matching lines @@
               "lib/browser/headless_permission_manager.cc",
               "lib/browser/headless_permission_manager.h",
               "lib/browser/headless_platform_event_source.cc",
               "lib/browser/headless_platform_event_source.h",
               "lib/browser/headless_shell_application_mac.mm",
               "lib/browser/headless_shell_application_mac.h",
               "lib/browser/headless_url_request_context_getter.cc",
               "lib/browser/headless_url_request_context_getter.h",
               "lib/browser/headless_web_contents_impl.cc",
               "lib/browser/headless_web_contents_impl.h",
-              "lib/browser/headless_window_tree_host.h",
               "lib/headless_crash_reporter_client.cc",
               "lib/headless_crash_reporter_client.h",
               "lib/headless_content_client.cc",
               "lib/headless_content_client.h",
               "lib/headless_content_main_delegate.cc",
               "lib/headless_content_main_delegate.h",
               "public/headless_browser.cc",
               "public/headless_browser.h",
               "public/headless_browser_context.h",
               "public/headless_devtools_client.h",
@@ skipping 42 matching lines @@
       "lib/browser/headless_browser_impl_aura.cc",
       "lib/browser/headless_clipboard.cc",
       "lib/browser/headless_clipboard.h",
       "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 = [
     ":gen_devtools_client_api",
     ":version_header",
     "//base",
     "//components/crash/content/browser",
     "//components/security_state/content",
     "//components/security_state/core",
@@ skipping 187 matching lines @@
 
 executable("headless_example") {
   sources = [
     "app/headless_example.cc",
   ]
 
   deps = [
     "//headless:headless_shell_lib",
   ]
 }