We have long had a presubmit that warns when a file lacks a suitable
license, but because this is a warning it can be ignored by the author
and is not visible to reviewers. Every failure that is not prevented
will occur, and indeed newly added files from this year (seen in
crrev.com/c/3706888 and crrev.com/c/3437860, and possibly others) are
missing valid licenses.

This change turns the warning into an error on newly added files.

Bug: 1361031
Change-Id: Ie17bd6e591918affd9d865a3bf2be6c2bf62d1ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3887721Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>