Add a multi-context sample.
This will be useful for multi-context trace testing.
Bug: angleproject:5133
Change-Id: Ib4b6aac3069bc88939c1f5c409b2e7ef95a1a539
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3313598
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
