Specifically, this change shows how it can be triggered:
https://chromium-review.googlesource.com/c/chromium/src/+/3433606

Calling Popen.wait() causes a __warningregistry__ key to be added to
context.

Bug: None
Change-Id: I7ceb453ffd1eb1ea689e909e3cc5c832df7e9b59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3433466
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>