Remove userAgent getter (#126)

If needed, the user agent could be fetched via querying
`navigator.userAgent` in page.

Closes #110

lib/NetworkManager.js

@@ -19,16 +19,14 @@ const helper = require('./helper');
 class NetworkManager extends EventEmitter {
   /**
    * @param {!Connection} client
-   * @param {string} userAgent
    */
-  constructor(client, userAgent) {
+  constructor(client) {
     super();
     this._client = client;
     this._requestInterceptor = null;
     /* @type {!Map<string, !Request>} */
     this._idToRequest = new Map();
     this._httpHeaders = {};
-    this._userAgent = userAgent;
 
     this._client.on('Network.requestWillBeSent', this._onRequestWillBeSent.bind(this));
     this._client.on('Network.requestIntercepted', this._onRequestIntercepted.bind(this));
@@ -61,17 +59,9 @@ class NetworkManager extends EventEmitter {
    * @return {!Promise}
    */
   async setUserAgent(userAgent) {
-    this._userAgent = userAgent;
     return this._client.send('Network.setUserAgentOverride', { userAgent });
   }
 
-  /**
-   * @return {string}
-   */
-  userAgent() {
-    return this._userAgent;
-  }
-
   /**
    * @param {?function(!InterceptedRequest)} interceptor
    * @return {!Promise}