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

chrome/browser/resources/extensions/extensions.js

Index: chrome/browser/resources/extensions/extensions.js
diff --git a/chrome/browser/resources/extensions/extensions.js b/chrome/browser/resources/extensions/extensions.js
index bc03c9b1ee040344569d3d918b0dbfe60115a13d..81096b8bcb00111ba7d93614e7604e063df00325 100644
--- a/chrome/browser/resources/extensions/extensions.js
+++ b/chrome/browser/resources/extensions/extensions.js
@@ -44,7 +44,7 @@ cr.define('extensions', function() {
       // 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;
       }