This allows inserted footers to be specified as either before
or after other potentially-present footers.

It has one slight behavior change (reflected in the tests):
If after_keys is specified but *doesn't match* any pre-existing
footers, then the behavior does *not* switch to "insert as early
as possible". The behavior switch only happens if the after_keys
actually match a footer.

R=iannucci@chromium.org

Bug: 710547
Change-Id: If557978fe9309785285056eb557acbdc87960bb2
Reviewed-on: https://chromium-review.googlesource.com/487606Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>