Do not leak event listeners on navigation

This patch:
- introduces helper.addEventListener/helper.removeEventListeners
  to simplify event management
- moves NavigatorWatchdog over to the helper.addEventListener to
  stop leaking event listeners

lib/helper.js

@@ -115,6 +115,26 @@ class Helper {
       return `"${text}"`;
     }
   }
+
+  /**
+   * @param {!EventEmitter} emitter
+   * @param {string} eventName
+   * @param {function(?)} handler
+   * @return {{emitter: !EventEmitter, eventName: string, handler: function(?)}}
+   */
+  static addEventListener(emitter, eventName, handler) {
+    emitter.on(eventName, handler);
+    return { emitter, eventName, handler };
+  }
+
+  /**
+   * @param {!Array<{emitter: !EventEmitter, eventName: string, handler: function(?)}>}
+   */
+  static removeEventListeners(listeners) {
+    for (let listener of listeners)
+      listener.emitter.removeListener(listener.eventName, listener.handler);
+    listeners.splice(0, listeners.length);
+  }
 }
 
 module.exports = Helper;