Append parameters to webstore URL to indicate launch source.

chrome/browser/resources/shared/js/util.js

 // Copyright (c) 2011 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.
 
 /**
  * The global object.
  * @type {!Object}
  */
 const global = this;
 
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         e.button,
         e.altKey,
         e.ctrlKey,
         e.metaKey,
         e.shiftKey
       ]);
       e.preventDefault();
     }
   }
 });
+
+/**
+ * Creates a new URL which is the old URL with a GET param of key=value.
+ * @param {string} url The base URL. There is not sanity checking on the URL so
+ *     it must be passed in a proper format.
+ * @param {string} key The key of the param.
+ * @param {string} value The value of the param.
+ * @return {string}
+ */
+function appendParam(url, key, value) {
+  var param = encodeURIComponent(key) + '=' + encodeURIComponent(value);
+
+  if (url.indexOf('?') == -1)
+    return url + '?' + param;
+  return url + '&' + param;
+}