PanProjectChecks is called twice, once from the root PRESUBMIT.py and
once from third_party/blink/PRESUBMIT.py. For the file-based checks a
series of filters avoids redundant reporting, but certain global checks
were being run twice, potentially causing duplicate reporting.

This adds an optional global_checks flag which can be set to false in
the blink PRESUBMIT.

Bug: 1309977
Change-Id: Ia43a7a0f29cf7b58ac9bab0e9aa374e9bf0155a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3611409
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Jesse McKenna <jessemckenna@google.com>