headless: Add Node.js example

headless/README.md

 # Headless Chromium
 
 Headless Chromium allows running Chromium in a headless/server environment.
 Expected use cases include loading web pages, extracting metadata (e.g., the
 DOM) and generating bitmaps from page contents -- using all the modern web
 platform features provided by Chromium and Blink.
 
 There are two ways to use Headless Chromium:
 
 ## Usage via the DevTools remote debugging protocol
 
 1. Start a normal Chrome binary with the `--headless` command line flag
 (Linux-only for now):
 
 ```
 $ chrome --headless --remote-debugging-port=9222 https://chromium.org
 ```
 
 Currently you'll also need to use `--disable-gpu` to avoid an error from a
 missing Mesa library.
 
 2. Navigate to `http://localhost:9222` in another browser to open the
 [DevTools](https://developer.chrome.com/devtools) interface or use a tool such
 as [Selenium](http://www.seleniumhq.org/) to drive the headless browser.
 
+## Usage from Node.js
+
+For example, the [chrome-remote-interface](https://github.com/cyrus-and/chrome-remote-interface)
+Node.js package can be used to extract a page's DOM like this:
+
+```
+const CDP = require('chrome-remote-interface');
+
+CDP((client) => {
+  // Extract used DevTools domains.
+  const {Page, Runtime} = client;
+
+  // Enable events on domains we are interested in.
+  Promise.all([
+    Page.enable()
+  ]).then(() => {
+    return Page.navigate({url: 'https://example.com'});
+  });
+
+  // Evaluate outerHTML after page has loaded.
+  Page.loadEventFired(() => {
+    Runtime.evaluate({expression: 'document.body.outerHTML'}).then((result) => {
+      console.log(result.result.value);
+      client.close();
+    });
+  });
+}).on('error', (err) => {
+  console.error('Cannot connect to browser:', err);
+});
+```
+
 ## Usage as a C++ library
 
 Headless Chromium can be built as a library for embedding into a C++
 application. This approach is otherwise similar to controlling the browser over
 a DevTools connection, but it provides more customization points, e.g., for
 networking and [mojo services](https://docs.google.com/document/d/1Fr6_DJH6OK9rG3-ibMvRPTNnHsAXPk0VzxxiuJDSK3M/edit#heading=h.qh0udvlk963d).
 
 [Headless Example](https://cs.chromium.org/chromium/src/headless/app/headless_example.cc)
 is a small sample application which demonstrates the use of the headless C++
 API. It loads a web page and outputs the resulting DOM. To run it, first
@@ skipping 64 matching lines @@
 * [Runtime headless mode for Chrome](https://docs.google.com/document/d/1aIJUzQr3eougZQp90bp4mqGr5gY6hdUice8UPa-Ys90/edit#)
 * [Virtual Time in Blink](https://docs.google.com/document/d/1y9kdt_zezt7pbey6uzvt1dgklwc1ob_vy4nzo1zbqmo/edit#heading=h.tn3gd1y9ifml)
 * [Headless Chrome architecture](https://docs.google.com/document/d/11zIkKkLBocofGgoTeeyibB2TZ_k7nR78v7kNelCatUE/edit)
 * [Headless Chrome C++ DevTools API](https://docs.google.com/document/d/1rlqcp8nk-ZQvldNJWdbaMbwfDbJoOXvahPCDoPGOwhQ/edit#heading=h.ng2bxb15li9a)
 * [Session isolation in Headless Chrome](https://docs.google.com/document/d/1XAKvrxtSEoe65vNghSWC5S3kJ--z2Zpt2UWW1Fi8GiM/edit)
 * [Headless Chrome mojo service](https://docs.google.com/document/d/1Fr6_DJH6OK9rG3-ibMvRPTNnHsAXPk0VzxxiuJDSK3M/edit#heading=h.qh0udvlk963d)
 * [Controlling BeginFrame through DevTools](https://docs.google.com/document/d/1LVMYDkfjrrX9PNkrD8pJH5-Np_XUTQHIuJ8IEOirQH4/edit?ts=57d96dbd#heading=h.ndv831lc9uf0)
 * [Viewport bounds and scale for screenshots](https://docs.google.com/document/d/1VTcYz4q_x0f1O5IVrvRX4u1DVd_K34IVUl1VULLTCWw/edit#heading=h.ndv831lc9uf0)
 * [BlinkOn 6 presentation slides](https://docs.google.com/presentation/d/1gqK9F4lGAY3TZudAtdcxzMQNEE7PcuQrGu83No3l0lw/edit#slide=id.p)
 * [Architecture design doc](https://docs.google.com/document/d/11zIkKkLBocofGgoTeeyibB2TZ_k7nR78v7kNelCatUE)