DevTools: update layout test docs for debugging DevTools tests

Bug: 667560
Change-Id: Ic335a8364393e85de100e5b82f2d03dd6983ff84
Reviewed-on: https://chromium-review.googlesource.com/801258Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Commit-Queue: Will Chen <chenwilliam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520699}

docs/testing/layout_tests.md

@@ -399,34 +399,35 @@ machine?
 * If none of that helps, and you have access to the bot itself, you may have to
   log in there and see if you can reproduce the problem manually.
 
-### Debugging Inspector Tests
-
-* Add `window.debugTest = true;` to your test code as follows:
-
-  ```javascript
-  window.debugTest = true;
-  function test() {
-    /* TEST CODE */
-  }
-  ```
+### Debugging DevTools Tests
 
+* Add `debug_devtools=true` to args.gn and compile: `ninja -C out/Default devtools_frontend_resources`
+  > Debug DevTools lets you avoid having to recompile after every change to the DevTools front-end.
 * Do one of the following:
     * Option A) Run from the chromium/src folder:
       `blink/tools/run_layout_tests.sh
-      --additional_driver_flag='--remote-debugging-port=9222'
+      --additional-driver-flag='--debug-devtools'
+      --additional-driver-flag='--remote-debugging-port=9222'
       --time-out-ms=6000000`
     * Option B) If you need to debug an http/tests/inspector test, start httpd
       as described above. Then, run content_shell:
-      `out/Default/content_shell --remote-debugging-port=9222 --run-layout-test
+      `out/Default/content_shell --debug-devtools --remote-debugging-port=9222 --run-layout-test
       http://127.0.0.1:8000/path/to/test.html`
 * Open `http://localhost:9222` in a stable/beta/canary Chrome, click the single
   link to open the devtools with the test loaded.
-* You may need to replace devtools.html with inspector.html in your URL (or you
-  can use local chrome inspection of content_shell from `chrome://inspect`
-  instead)
 * In the loaded devtools, set any required breakpoints and execute `test()` in
   the console to actually start the test.
 
+NOTE: If the test is an html file, this means it's a legacy test so you need to add:
+* Add `window.debugTest = true;` to your test code as follows:
+
+  ```javascript
+  window.debugTest = true;
+  function test() {
+    /* TEST CODE */
+  }
+  ```  
+
 ## Bisecting Regressions
 
 You can use [`git bisect`](https://git-scm.com/docs/git-bisect) to find which