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

chrome/browser/extensions/process_management_browsertest.cc

 // Copyright (c) 2012 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 <stddef.h>
 
 #include "base/strings/utf_string_conversions.h"
 #include "build/build_config.h"
 #include "chrome/browser/chrome_notification_types.h"
 #include "chrome/browser/extensions/extension_apitest.h"
 #include "chrome/browser/extensions/extension_service.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_manager.h"
 #include "chrome/browser/sessions/session_tab_helper.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
 #include "chrome/common/chrome_switches.h"
-#include "chrome/common/extensions/extension_process_policy.h"
 #include "chrome/common/url_constants.h"
 #include "chrome/test/base/ui_test_utils.h"
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/render_frame_host.h"
 #include "content/public/browser/render_process_host.h"
 #include "content/public/browser/render_view_host.h"
 #include "content/public/browser/site_instance.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/test/browser_test_utils.h"
 #include "content/public/test/test_navigation_observer.h"
@@ skipping 412 matching lines @@
   // the process is the same for all SiteInstances. This allows us to verify
   // that the site-to-process map for the extension hasn't been overwritten by
   // the process of the |blocked_url|.
   scoped_refptr<content::SiteInstance> new_site_instance =
       content::SiteInstance::CreateForURL(web_contents->GetBrowserContext(),
                                           extension->GetResourceURL(""));
   EXPECT_TRUE(new_site_instance->HasProcess());
   EXPECT_EQ(new_site_instance->GetProcess(),
             web_contents->GetSiteInstance()->GetProcess());
 }