Document more topics about running/debugging Content Shell

- The protocol mode
- Running HTTP web tests in Content Shell
- Running WPT tests in Content Shell
- Debugging with --single-process
- Debugging web tests
- Some corrections

Change-Id: I1d23d0f8a75b9d597edab153e4ec0ea9cdb7cb67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1541863
Commit-Queue: Xianzhu Wang <[email protected]>
Reviewed-by: Robert Ma <[email protected]>
Cr-Commit-Position: refs/heads/master@{#645002}
diff --git a/docs/testing/web_platform_tests.md b/docs/testing/web_platform_tests.md
index 6457a66..83e4df9e 100644
--- a/docs/testing/web_platform_tests.md
+++ b/docs/testing/web_platform_tests.md
@@ -249,6 +249,9 @@
 
 One thing to note is that glob patterns for WPT tests are not yet supported.
 
+See [Running WPT tests in Content Shell](web_tests_in_content_shell.md#Running-WPT-Tests-in-Content-Shell)
+for debugging etc.
+
 ## Reviewing tests
 
 Anyone who can review code and tests in Chromium can also review changes in