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

chrome/browser/extensions/chrome_content_browser_client_extensions_part.cc

 // 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.
 
 #include "chrome/browser/extensions/chrome_content_browser_client_extensions_part.h"
 
 #include <stddef.h>
 
 #include <set>
 
 #include "base/command_line.h"
 #include "base/debug/alias.h"
 #include "base/debug/dump_without_crashing.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/extensions/extension_service.h"
 #include "chrome/browser/extensions/extension_web_ui.h"
 #include "chrome/browser/extensions/extension_webkit_preferences.h"
 #include "chrome/browser/media_galleries/fileapi/media_file_system_backend.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_io_data.h"
 #include "chrome/browser/profiles/profile_manager.h"
 #include "chrome/browser/renderer_host/chrome_extension_message_filter.h"
 #include "chrome/browser/sync_file_system/local/sync_file_system_backend.h"
 #include "chrome/common/chrome_constants.h"
 #include "chrome/common/chrome_switches.h"
 #include "chrome/common/extensions/extension_constants.h"
-#include "chrome/common/extensions/extension_process_policy.h"
 #include "chrome/common/url_constants.h"
 #include "components/dom_distiller/core/url_constants.h"
 #include "components/guest_view/browser/guest_view_message_filter.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/browser_url_handler.h"
 #include "content/public/browser/page_navigator.h"
 #include "content/public/browser/render_process_host.h"
 #include "content/public/browser/render_view_host.h"
 #include "content/public/browser/resource_dispatcher_host.h"
 #include "content/public/browser/site_instance.h"
@@ skipping 829 matching lines @@
     command_line->AppendSwitch(switches::kExtensionProcess);
   }
 }
 
 void ChromeContentBrowserClientExtensionsPart::ResourceDispatcherHostCreated() {
   content::ResourceDispatcherHost::Get()->RegisterInterceptor(
       "Origin", kExtensionScheme, base::Bind(&OnHttpHeaderReceived));
 }
 
 }  // namespace extensions