wptserve: document hardcoded gen dir

run_blink_wptserve.py has no way of knowing which gen/ directory
to serve files from. Document this fact.

R=qyearsley@chromium.org

Change-Id: Ia5caef49b7333e695665ca921790c4dedb30ac22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2217275
Commit-Queue: Johann Koenig <johannkoenig@google.com>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774058}

docs/testing/web_tests_in_content_shell.md

@@ -94,6 +94,14 @@ tests require some setup before running in Content Shell:
 ```bash
 python third_party/blink/tools/run_blink_wptserve.py
 ```
+
+run_blink_wptserve.py serves generated content from out/Release/gen. If you are
+using a different build directory it can be symlinked:
+
+```bash
+ln -s /path/to/out/Build/gen /path/to/out/Release/gen
+```
+
 Then run the test:
 
 ```bash

third_party/blink/tools/run_blink_wptserve.py

@@ -16,4 +16,9 @@ from blinkpy.common import version_check  # pylint: disable=unused-import
 from blinkpy.web_tests.servers import cli_wrapper
 from blinkpy.web_tests.servers import wptserve
 
+print(
+    "Generated content (out/BUILD_DIR/gen) is *only* served from "
+    "out/Release/gen. It is possible to symlink a gen dir from another build "
+    "directory.")
+
 cli_wrapper.main(wptserve.WPTServe, description=__doc__)