Allow user script install by dragging onto chrome://extensions/.

chrome/browser/resources/extensions/extensions.js

 // 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 src="../shared/js/cr/ui/drag_wrapper.js"></include>
 <include src="../uber/uber_utils.js"></include>
 <include src="extension_commands_overlay.js"></include>
 <include src="extension_list.js"></include>
 <include src="pack_extension_overlay.js"></include>
 
@@ skipping 26 matching lines @@
       chrome.send('stopDrag');
     },
     // @inheritdoc
     doDragOver: function(e) {
     },
     // @inheritdoc
     doDrop: function(e) {
       // Only process files that look like extensions. Other files should
       // navigate the browser normally.
       if (!e.dataTransfer.files.length ||
-          !/\.crx$/.test(e.dataTransfer.files[0].name)) {
+          !/\.(crx|user\.js)$/.test(e.dataTransfer.files[0].name)) {
         return;
       }
 
       chrome.send('installDroppedFile');
       ExtensionSettings.showOverlay(null);
       e.preventDefault();
     }
   };
 
   /**
@@ skipping 253 matching lines @@
   return {
     ExtensionSettings: ExtensionSettings
   };
 });
 
 var ExtensionSettings = extensions.ExtensionSettings;
 
 window.addEventListener('load', function(e) {
   ExtensionSettings.getInstance().initialize();
 });