docs: Add [TOC] to web_tests_in_content_shell.md

TBR=robertma

Change-Id: Id6b2205e0e0008737655eb689d0814a1f8c6c67a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2566996Reviewed-by: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#832378}

docs/testing/web_tests_in_content_shell.md

 # Running web tests using the content shell
 
+[TOC]
+
 ## Compiling
 
 If you want to run web tests,