For bonus points, this also fixes PRESUBMIT so that it actually checks
the manifests.

TBR=bpastene@chromium.org, tandrii@chromium.org

Bug: 1032743
Change-Id: I4dac5a520e333e3c429597684a44ece83d21c5fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1960950Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>