Use PreloadCheckGroup in ExtensionInstallChecker. Make RequirementsChecker a PreloadCheck.

chrome/browser/extensions/unpacked_installer.h

Index: chrome/browser/extensions/unpacked_installer.h
diff --git a/chrome/browser/extensions/unpacked_installer.h b/chrome/browser/extensions/unpacked_installer.h
index c0416a20bdfaf4d064afbfc91cdb926ca3e98aed..2eeedb540d3d7b2a5cb37f19177285344e823b70 100644
--- a/chrome/browser/extensions/unpacked_installer.h
+++ b/chrome/browser/extensions/unpacked_installer.h
@@ -117,8 +117,6 @@ class UnpackedInstaller
   // 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_;
 
@@ -126,6 +124,9 @@ class UnpackedInstaller
   // 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_;