presubmit_support: Prevent crash when checks use Popen
presubmit_support: Prevent crash when checks use Popen

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 <f5a68dd3fb158cd420e05ca279f52a686c662bee@chromium.org>
Reviewed-by: Dirk Pranke <6693ee08b084bb2ffa767f664c99b689395e0e4b@google.com>
Commit-Queue: Dirk Pranke <6693ee08b084bb2ffa767f664c99b689395e0e4b@google.com>

CoherentLabs/depot_tools,CoherentLabs/depot_tools