headless: Replace is_headless gn setting with an args template

headless/README.md

Index: headless/README.md
diff --git a/headless/README.md b/headless/README.md
index 54e9d0ed591971948da16b0d0b2c643d7bb5c834..0748cfd8f6a330771a40315d26096ecb32ea06ec 100644
--- a/headless/README.md
+++ b/headless/README.md
@@ -8,32 +8,32 @@ 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/Debug
+$ echo 'import("//build/args/headless.gn")' > out/Debug/args.gn
+$ gn gen out/Debug
 ```
 
-and enable headless mode with `is_headless = true`.
-
 Then build the shell:
 
 ```
-$ ninja -C out/Release headless_shell
+$ ninja -C out/Debug headless_shell
 ```
 
 After the build completes, the headless shell can be run with the following
 command:
 
 ```
-$ out/Release/headless_shell https://www.google.com
+$ 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/Release/headless_shell --remote-debugging-port=9222 https://youtube.com
+$ out/Debug/headless_shell --remote-debugging-port=9222 https://youtube.com
 ```
 
 Then navigate to `http://127.0.0.1:9222` with your browser.