With --isolate-extensions, forking (via OpenURL) for extensions is not needed.

chrome/common/BUILD.gn

 # Copyright 2014 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/buildflag_header.gni")
 import("//build/util/process_version.gni")
 import("//chrome/common/features.gni")
 import("//chrome/process_version_rc_template.gni")  # For branding_file_path.
 import("//extensions/features/features.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
@@ skipping 266 matching lines @@
       "extensions/chrome_manifest_handlers.h",
       "extensions/chrome_manifest_url_handlers.cc",
       "extensions/chrome_manifest_url_handlers.h",
       "extensions/chrome_utility_extensions_messages.h",
       "extensions/command.cc",
       "extensions/command.h",
       "extensions/extension_constants.cc",
       "extensions/extension_constants.h",
       "extensions/extension_metrics.cc",
       "extensions/extension_metrics.h",
-      "extensions/extension_process_policy.cc",
-      "extensions/extension_process_policy.h",
       "extensions/image_writer/image_writer_util_mac.cc",
       "extensions/image_writer/image_writer_util_mac.h",
       "extensions/manifest_handlers/app_icon_color_info.cc",
       "extensions/manifest_handlers/app_icon_color_info.h",
       "extensions/manifest_handlers/app_launch_info.cc",
       "extensions/manifest_handlers/app_launch_info.h",
       "extensions/manifest_handlers/automation.cc",
       "extensions/manifest_handlers/automation.h",
       "extensions/manifest_handlers/content_scripts_handler.cc",
       "extensions/manifest_handlers/content_scripts_handler.h",
@@ skipping 432 matching lines @@
 mojom("instant_mojom") {
   sources = [
     "instant.mojom",
   ]
 
   public_deps = [
     "//mojo/common:common_custom_types",
     "//url/mojo:url_mojom_gurl",
   ]
 }