headless: Replace is_headless gn setting with an args template

headless/README.md

 # Headless Chromium
 
 Headless Chromium is a library for 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.
 
 ## Headless shell
 
 The headless shell is a sample application which demonstrates the use of the
-headless API. To run it, first open the build configuration editor:
+headless API. To run it, first initialize a headless build configuration:
 
 ```
-$ gn args out/Release
+$ mkdir -p out/Release
+$ echo 'import("//build/args/headless.gn")' > out/Release/args.gn
+$ gn gen out/Release

[Dirk Pranke, 2016/03/31 01:47:30]

//build/args/headless.gn in this patchset will produce a debug build
(is_debug=true by default), so you should probably either change that or change
this doc to out/Debug (or out/Default).

[Sami, 2016/03/31 09:43:02]

Ah, good point. Changed this to talk about debug instead.

 ```
 
-and enable headless mode with `is_headless = true`.
-
 Then build the shell:
 
 ```
 $ ninja -C out/Release headless_shell
 ```
 
 After the build completes, the headless shell can be run with the following
 command:
 
 ```
@@ skipping 23 matching lines @@
   - `SetProxyServer` - Configures an HTTP/HTTPS proxy server to be used for
     accessing the network.
 
 ## Client API
 
 The headless client API is used to drive the browser and interact with loaded
 web pages. Its main classes are:
 
 - `HeadlessBrowser` - Represents the global headless browser instance.
 - `HeadlessWebContents` - Represents a single "tab" within the browser.