Add support for running web tests with debug_renderer

If debug_renderer detects that you're running web tests, modifies the
command line flags and adds appropriate run_web_tests flags for
debugging the renderer spawned for individual web tests.

Bug: None
Change-Id: I85980ff57c85738f8ad8d47e83ba9d61d5411e09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4198115
Commit-Queue: Robert Flack 
Reviewed-by: Xianzhu Wang 
Cr-Commit-Position: refs/heads/main@{#1109686}
diff --git a/docs/linux/debugging.md b/docs/linux/debugging.md
index 587080a..d0251ea2 100644
--- a/docs/linux/debugging.md
+++ b/docs/linux/debugging.md
@@ -145,6 +145,16 @@
 gdb -p 
 ```
 
+#### Debugging run_web_tests.py renderers
+
+The `debug_renderer` script can also be used to debug the renderer running
+a web test. To do so, simply call `run_web_tests.py` from `debug_renderer.py`
+with all of the standard arguments for `run_web_tests.py`. For example:
+
+```sh
+./third_party/blink/tools/debug_renderer ./third_party/blink/tools/run_web_tests.py [run_web_test args]
+```
+
 #### Selective breakpoints
 
 When debugging both the browser and renderer process, you might want to have