Use PreloadCheckGroup in ExtensionInstallChecker. Make RequirementsChecker a PreloadCheck.

chrome/browser/extensions/unpacked_installer.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_BROWSER_EXTENSIONS_UNPACKED_INSTALLER_H_
 #define CHROME_BROWSER_EXTENSIONS_UNPACKED_INSTALLER_H_
 
 #include <memory>
 #include <string>
 #include <vector>
@@ skipping 99 matching lines @@
 
   // Notify the frontend that there was an error loading an extension.
   void ReportExtensionLoadError(const std::string& error);
 
   // Passes the extension onto extension service.
   void InstallExtension();
 
   // Helper to get the Extension::CreateFlags for the installing extension.
   int GetFlags();
 
-  const Extension* extension() { return install_checker_.extension().get(); }
-
   // The service we will report results back to.
   base::WeakPtr<ExtensionService> service_weak_;
 
   // The pathname of the directory to load from, which is an absolute path
   // after GetAbsolutePath has been called.
   base::FilePath extension_path_;
 
+  // The extension being installed.
+  scoped_refptr<const Extension> extension_;
+
   // If true and the extension contains plugins, we prompt the user before
   // loading.
   bool prompt_for_plugins_;
 
   // Whether to require the extension installed to have a modern manifest
   // version.
   bool require_modern_manifest_version_;
 
   // Whether or not to be noisy (show a dialog) on failure. Defaults to true.
   bool be_noisy_on_failure_;
 
   // Checks management policies and requirements before the extension can be
   // installed.
   ExtensionInstallChecker install_checker_;
 
   CompletionCallback callback_;
 
   DISALLOW_COPY_AND_ASSIGN(UnpackedInstaller);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_UNPACKED_INSTALLER_H_