After optimizing CheckForIncludeGuards the CheckLicense presubmit was
the second slowest presubmit, taking about 35 minutes when run as part
of "git cl presubmit --all". The cause of its slowness was initially
non-obvious because it runs quite quickly on most files, however
analysis showed that it could take 50+ seconds on some files. The files
that it is slow on are those that lack a good license header, meaning
that the regex match has to painstakingly scan the entire file.

The optimization in this change is to recognize that there is a simple
non-regex line that appears in all valid license headers, regardless of
variants. If that line is absent then there is, necessarily, no valid
license header, and searching for a line of text is something that
Python can do extremely quickly.

This change drops the CheckLicense time from about 35 minutes to about
32 seconds.

Trivia: _CommonChecks in third_party/blink/PRESUBMIT.py passes .* as the
license, so I added an early-out for that to avoid pointlessly scanning
those files.

Bug: 1309977
Change-Id: Ic2e56079675c2c5a2643d20dd492b1cc52e4ead2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3584882Reviewed-by: Erik Staab <estaab@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>