docs: specify run_blink_httpd.py directory

Most of the files served by run_blink_httpd.py are from the git
checkout. A few are not, so make sure developers know they can
specify the checkout directory with -t.

Change-Id: If66ebce1db41e05e46e792e310838ee0ed2741ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2405715
Auto-Submit: Johann Koenig <johannkoenig@google.com>
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806172}

docs/testing/web_tests_in_content_shell.md

@@ -78,10 +78,11 @@ On Windows, use `content_shell.exe`.
 #### Running HTTP Tests in Content Shell
 
 HTTP tests reside under [web_tests/http/tests](../../third_party/blink/web_tests/http/tests).
-You need to start a web server first:
+You need to start a web server first. By default it serves generated files from
+out/Release:
 
 ```bash
-python third_party/blink/tools/run_blink_httpd.py
+python third_party/blink/tools/run_blink_httpd.py -t <build directory>
 ```
 Then run the test with a localhost URL: