Misc cleanup of SecurityContext interface

- Make InsecureNavigationsToUpgrade() return a reference to
  ensure it's always non-null.
- Add some const.
- Remove virtual where not overridden.

Bug: 878274
Change-Id: I4807d62a2d0f7e792cac46016f2c120fb7aebb6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1580144
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653881}