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

chrome/browser/extensions/process_manager_browsertest.cc

 // Copyright 2013 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 <memory>
 #include <utility>
 
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/path_service.h"
 #include "base/run_loop.h"
 #include "base/test/histogram_tester.h"
 #include "chrome/browser/chrome_notification_types.h"
 #include "chrome/browser/extensions/browser_action_test_util.h"
 #include "chrome/browser/extensions/extension_browsertest.h"
 #include "chrome/browser/extensions/extension_service.h"
 #include "chrome/browser/extensions/test_extension_dir.h"
 #include "chrome/browser/ui/browser_commands.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
 #include "chrome/common/chrome_paths.h"
-#include "chrome/common/extensions/extension_process_policy.h"
 #include "chrome/common/pref_names.h"
 #include "chrome/test/base/in_process_browser_test.h"
 #include "chrome/test/base/ui_test_utils.h"
 #include "components/guest_view/browser/test_guest_view_manager.h"
 #include "content/public/browser/child_process_security_policy.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/web_contents.h"
 #include "content/public/common/browser_side_navigation_policy.h"
@@ skipping 1209 matching lines @@
   GURL b_url(embedded_test_server()->GetURL("b.com", "/empty.html"));
   EXPECT_TRUE(content::NavigateIframeToURL(tab, "frame0", b_url));
   {
     content::RenderFrameHost* subframe = ChildFrameAt(main_frame, 0);
     EXPECT_EQ(subframe->GetProcess(), main_frame->GetProcess());
     EXPECT_EQ(subframe->GetSiteInstance(), main_frame->GetSiteInstance());
   }
 }
 
 }  // namespace extensions