We want PRESUBMIT to be able to equally support
BUG= tags (old style) and Git-Footer: footers
(new style). This change refactors the way that
the presubmit api gives access to those properties
so that it is easier to add support for equivalent
footers.

It also limits the scope of tags/footers that it
exposes, as code search shows no PRESUBMIT files
that take advantage of any of the more esoteric
ones.

Bug: 710327, 710803
Change-Id: I86f1d6cb2e1f0aff9653ef3fb455e0a6f47acf5d
Reviewed-on: https://chromium-review.googlesource.com/506450
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>