Only load u2f-api.js implementation when necessary

Some browsers such as Firefox already provide a window.u2f
implementation. Detect the existing implementation and abort from
u2f-api.js.

src/js/u2f-api.js

@@ -11,12 +11,28 @@
  */
 'use strict';
 
+/**
+ * Modification:
+ * Wrap implementation so that we can exit if window.u2f is already supplied by the browser (see below).
+ */
+(function (root) {
+    /**
+     * Modification:
+     * Only continue load this library if window.u2f is not already supplied by the browser.
+     */
+    var isFirefox = navigator.userAgent.indexOf('Firefox') !== -1 || navigator.userAgent.indexOf('Gecko/') !== -1;
+    var browserImplementsU2f = !!((typeof root.u2f !== 'undefined') && root.u2f.register);
+
+    if (isFirefox && browserImplementsU2f) {
+        root.u2f.isSupported = true;
+        return;
+    }
 
     /**
      * Namespace for the U2F api.
      * @type {Object}
      */
-var u2f = u2f || {};
+    var u2f = root.u2f || {};
 
     /**
      * Modification:
@@ -754,3 +770,10 @@ u2f.getApiVersion = function (callback, opt_timeoutSeconds) {
             port.postMessage(req);
         });
     };
+
+    /**
+     * Modification:
+     * Assign u2f back to window (root) scope.
+     */
+    root.u2f = u2f;
+}(this));