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

chrome/browser/extensions/window_open_apitest.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/path_service.h"
 #include "base/strings/stringprintf.h"
 #include "base/test/histogram_tester.h"
 #include "build/build_config.h"
@@ skipping 244 matching lines @@
 // Tests that if an extension page calls window.open to an invalid extension
 // URL, the browser doesn't crash.
 IN_PROC_BROWSER_TEST_F(ExtensionBrowserTest, WindowOpenInvalidExtension) {
   ASSERT_TRUE(LoadExtension(
       test_data_dir_.AppendASCII("uitest").AppendASCII("window_open")));
 
   GURL start_url(std::string(extensions::kExtensionScheme) +
                      url::kStandardSchemeSeparator +
                      last_loaded_extension_id() + "/test.html");
   ui_test_utils::NavigateToURL(browser(), start_url);
-  ASSERT_NO_FATAL_FAILURE(
-      OpenWindow(browser()->tab_strip_model()->GetActiveWebContents(),
+  ASSERT_NO_FATAL_FAILURE(OpenWindow(
+      browser()->tab_strip_model()->GetActiveWebContents(),
       GURL("chrome-extension://thisissurelynotavalidextensionid/newtab.html"),
-      false, NULL));
+      true, NULL));
 
   // If we got to this point, we didn't crash, so we're good.
 }
 
 // Tests that calling window.open from the newtab page to an extension URL
 // gives the new window extension privileges - even though the opening page
 // does not have extension privileges, we break the script connection, so
 // there is no privilege leak.
 IN_PROC_BROWSER_TEST_F(ExtensionBrowserTest, WindowOpenNoPrivileges) {
   ASSERT_TRUE(LoadExtension(
       test_data_dir_.AppendASCII("uitest").AppendASCII("window_open")));
 
   ui_test_utils::NavigateToURL(browser(), GURL("about:blank"));
   WebContents* newtab = NULL;
   ASSERT_NO_FATAL_FAILURE(
       OpenWindow(browser()->tab_strip_model()->GetActiveWebContents(),
                  GURL(std::string(extensions::kExtensionScheme) +
-                     url::kStandardSchemeSeparator +
-                     last_loaded_extension_id() + "/newtab.html"),
-                 false,
-                 &newtab));
+                      url::kStandardSchemeSeparator +
+                      last_loaded_extension_id() + "/newtab.html"),
+                 true, &newtab));
 
   // Extension API should succeed.
   bool result = false;
   ASSERT_TRUE(content::ExecuteScriptAndExtractBool(newtab, "testExtensionApi()",
                                                    &result));
   EXPECT_TRUE(result);
 }
 
 // Tests that calling window.open for an extension URL from a non-HTTP or HTTPS
 // URL on a new tab cannot access non-web-accessible resources.
@@ skipping 62 matching lines @@
     ASSERT_TRUE(content::ExecuteScript(
         tab, "location.href = '" + extension_url.spec() + "';"));
     observer.Wait();
     EXPECT_EQ(extension_url, tab->GetMainFrame()->GetLastCommittedURL());
     std::string result;
     ASSERT_TRUE(content::ExecuteScriptAndExtractString(
         tab, "domAutomationController.send(document.body.innerText)", &result));
     EXPECT_EQ("HOWDIE!!!", result);
   }
 }