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

chrome/browser/site_details_browsertest.cc

 // 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.
 
 #include "chrome/browser/site_details.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <memory>
 #include <utility>
 #include <vector>
 
 #include "base/bind_helpers.h"
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/message_loop/message_loop.h"
 #include "base/path_service.h"
 #include "base/strings/stringprintf.h"
 #include "base/test/histogram_tester.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/extensions/extension_browsertest.h"
 #include "chrome/browser/extensions/test_extension_dir.h"
 #include "chrome/browser/metrics/metrics_memory_details.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
-#include "chrome/common/extensions/extension_process_policy.h"
 #include "chrome/common/url_constants.h"
 #include "chrome/test/base/in_process_browser_test.h"
 #include "chrome/test/base/ui_test_utils.h"
 #include "components/metrics/metrics_service.h"
 #include "components/variations/metrics_util.h"
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/render_process_host.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/test/browser_test_utils.h"
 #include "content/public/test/test_utils.h"
@@ skipping 1180 matching lines @@
   content::NavigateIframeToURL(
       tab, "child-1", extension2->GetResourceURL("/blank_iframe.html"));
   details = new TestMemoryDetails();
   details->StartFetchAndWait();
   EXPECT_THAT(details->uma()->GetAllSamples(
                   "SiteIsolation.SiteInstancesPerBrowsingInstance"),
               DependingOnPolicy(ElementsAre(Bucket(1, 2)),
                                 ElementsAre(Bucket(1, 1), Bucket(3, 1)),
                                 ElementsAre(Bucket(1, 1), Bucket(5, 1))));
 }