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

chrome/common/extensions/extension_process_policy.h

-// 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.
-
-#ifndef CHROME_COMMON_EXTENSIONS_EXTENSION_PROCESS_POLICY_H_
-#define CHROME_COMMON_EXTENSIONS_EXTENSION_PROCESS_POLICY_H_
-
-class GURL;
-
-namespace extensions {
-
-class Extension;
-class ExtensionSet;
-
-// Returns the extension for the given URL.  Excludes extension objects for
-// bookmark apps, which do not use the app process model.
-const Extension* GetNonBookmarkAppExtension(const ExtensionSet& extensions,
-                                            const GURL& url);
-
-// Check if navigating a toplevel page from |old_url| to |new_url| would cross
-// an extension process boundary (e.g. navigating from a web URL into an
-// extension URL).
-// We temporarily consider a workaround where we will keep non-app URLs in
-// an app process, but only if |should_consider_workaround| is true.  See
-// http://crbug.com/59285.
-bool CrossesExtensionProcessBoundary(
-    const ExtensionSet& extensions,
-    const GURL& old_url,
-    const GURL& new_url,
-    bool should_consider_workaround);
-
-}  // namespace extensions
-
-#endif  // CHROME_COMMON_EXTENSIONS_EXTENSION_PROCESS_POLICY_H_