headless: Add a small C++ example application

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
@@ skipping 12 matching lines @@
 [DevTools](https://developer.chrome.com/devtools) interface or use a tool such
 as [Selenium](http://www.seleniumhq.org/) to drive the headless browser.
 
 ## 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 Shell is a sample application which demonstrates the use of the
-headless C++ API. To run it, first initialize a headless build configuration:
+[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
+initialize a headless build configuration:
 
 ```
 $ mkdir -p out/Debug
 $ echo 'import("//build/args/headless.gn")' > out/Debug/args.gn
 $ gn gen out/Debug
 ```
 
-Then build the shell:
+Then build the example:
+
+```
+$ ninja -C out/Debug headless_example
+```
+
+After the build completes, the example can be run with the following command:
+
+```
+$ out/Debug/headless_example https://www.google.com
+```
+
+[Headless Shell](https://cs.chromium.org/chromium/src/headless/app/headless_shell.cc)
+is a more capable headless application. For instance, it supports remote
+debugging with the [DevTools](https://developer.chrome.com/devtools) protocol.
+To do this, start the application with an argument specifying the debugging
+port:
 
 ```
 $ ninja -C out/Debug headless_shell
-```
-
-After the build completes, Headless Shell can be run with the following command:
-
-```
-$ out/Debug/headless_shell https://www.google.com
-```
-
-To attach a [DevTools](https://developer.chrome.com/devtools) debugger to the
-shell, start it with an argument specifying the debugging port:
-
-```
 $ out/Debug/headless_shell --remote-debugging-port=9222 https://youtube.com
 ```
 
 Then navigate to `http://localhost:9222` with your browser.
 
 ## Embedder API
 
 The embedder API allows developers to integrate the headless library into their
 application. The API provides default implementations for low level adaptation
 points such as networking and the run loop.
@@ skipping 26 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)