GitLab

This requires adding a PRESUBMIT.py to /testing/scripts to run the unit tests.

Includes some small updates to wpt_common, mainly:
- ensuring crashlot artifact path is added to output json
- replacing os.path.exists with FileSystem.exists

Bug: 1131478

Change-Id: Ia470f57b43709d20825d7f91017647b85d71ad48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422346
Commit-Queue: Luke Z <lpz@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809757}