feat(Page): introduce Page bringToFront method (puppeteer#1252)

This patch introduces Page.bringToFront method to activate tabs.

Fixes puppeteer#1244

docs/api.md

@@ -45,6 +45,7 @@
   * [page.addScriptTag(options)](#pageaddscripttagoptions)
   * [page.addStyleTag(options)](#pageaddstyletagoptions)
   * [page.authenticate(credentials)](#pageauthenticatecredentials)
+  * [page.bringToFront()](#pagebringtofront)
   * [page.click(selector[, options])](#pageclickselector-options)
   * [page.close()](#pageclose)
   * [page.content()](#pagecontent)
@@ -524,6 +525,12 @@ Provide credentials for [http authentication](https://developer.mozilla.org/en-U
 
 To disable authentication, pass `null`.
 
+#### page.bringToFront()
+
+- returns: <[Promise]>
+
+Brings page to front (activates tab).
+
 #### page.click(selector[, options])
 - `selector` <[string]> A [selector] to search for element to click. If there are multiple elements satisfying the selector, the first will be clicked.
 - `options` <[Object]>

lib/Page.js

@@ -544,6 +544,10 @@ class Page extends EventEmitter {
     return response;
   }
 
+  async bringToFront() {
+    await this._client.send('Page.bringToFront');
+  }
+
   /**
    * @param {!Object} options
    */

test/test.js

@@ -3118,7 +3118,7 @@ describe('Page', function() {
 if (process.env.COVERAGE) {
   describe('COVERAGE', function(){
     const coverage = helper.publicAPICoverage();
-    const disabled = new Set();
+    const disabled = new Set(['page.bringToFront']);
     if (!headless)
       disabled.add('page.pdf');