feat: introduce browser.userAgent() (puppeteer#1716)

The patch introduces browser.userAgent() method to retrieve
default browser user agent.

Fixes puppeteer#1706.

docs/api.md

@@ -24,6 +24,7 @@
   * [browser.pages()](#browserpages)
   * [browser.process()](#browserprocess)
   * [browser.targets()](#browsertargets)
+  * [browser.userAgent()](#browseruseragent)
   * [browser.version()](#browserversion)
   * [browser.wsEndpoint()](#browserwsendpoint)
 - [class: Page](#class-page)
@@ -362,6 +363,11 @@ Disconnects Puppeteer from the browser, but leaves the Chromium process running.
 #### browser.targets()
 - returns: <[Array]<[Target]>> An array of all active targets.
 
+#### browser.userAgent()
+- returns: <[Promise]<[string]>> Promise which resolves to the browser's original user agent.
+
+> **NOTE** Pages can override browser user agent with [page.setUserAgent](#pagesetuseragentuseragent)
+
 #### browser.version()
 - returns: <[Promise]<[string]>> For headless Chromium, this is similar to `HeadlessChrome/61.0.3153.0`. For non-headless, this is similar to `Chrome/61.0.3153.0`.
 

lib/Browser.js

@@ -131,10 +131,18 @@ class Browser extends EventEmitter {
    * @return {!Promise<string>}
    */
   async version() {
-    const version = await this._connection.send('Browser.getVersion');
+    const version = await this._getVersion();
     return version.product;
   }
 
+  /**
+   * @return {!Promise<string>}
+   */
+  async userAgent() {
+    const version = await this._getVersion();
+    return version.userAgent;
+  }
+
   async close() {
     await this._closeCallback.call(null);
     this.disconnect();
@@ -143,6 +151,13 @@ class Browser extends EventEmitter {
   disconnect() {
     this._connection.dispose();
   }
+
+  /**
+   * @return {!Promise<!Object>}
+   */
+  _getVersion() {
+    return this._connection.send('Browser.getVersion');
+  }
 }
 
 /** @enum {string} */

test/test.js

@@ -273,6 +273,14 @@ describe('Page', function() {
     });
   });
 
+  describe('Browser.userAgent', function() {
+    it('should include WebKit', async({browser}) => {
+      const userAgent = await browser.userAgent();
+      expect(userAgent.length).toBeGreaterThan(0);
+      expect(userAgent).toContain('WebKit');
+    });
+  });
+
   describe('Browser.process', function() {
     it('should return child_process instance', async function({browser}) {
       const process = await browser.process();