Improve the UI for disabling off-store extension install.

chrome/browser/download/download_crx_util.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.
 //
 // Download code which handles CRX files (extensions, themes, apps, ...).
 
 #include "chrome/browser/download/download_util.h"
 #include "chrome/browser/extensions/crx_installer.h"
 #include "chrome/browser/extensions/extension_install_ui.h"
 #include "chrome/browser/extensions/extension_service.h"
@@ skipping 69 matching lines @@
     if (is_gallery_download)
       installer->set_original_download_url(download_item.GetOriginalUrl());
     installer->set_allow_silent_install(is_gallery_download);
     installer->set_install_cause(extension_misc::INSTALL_CAUSE_USER_DOWNLOAD);
     installer->InstallCrx(download_item.GetFullPath());
   }
 
   return installer;
 }
 
+bool IsExtensionDownload(const DownloadItem& download_item) {
+  if (download_item.PromptUserForSaveLocation())
+    return false;
+
+  if (download_item.GetMimeType() == extensions::Extension::kMimeType ||
+      UserScript::IsURLUserScript(download_item.GetURL(),
+                                  download_item.GetMimeType())) {
+    return true;
+  } else {
+    return false;
+  }
+}
+
 }  // namespace download_crx_util