[LayoutNG] Mark LayoutNGMixin functions as final when possible

Some override functions in LayoutNGMixin are not to be
overridden further. This patch marks them as 'final' so that
calling them from inside of LayoutNGMixin or its subclasses
can make non-virtual calls.

No behavior changes.

Bug: 635619
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I97e65d70cbe34b6757ab129c62e37bdaddcda72e
Reviewed-on: https://chromium-review.googlesource.com/1258664
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596285}
3 files changed