[turbofan] Cleanup use of virtual, OVERRIDE, FINAL.

[turbofan] Cleanup use of virtual, OVERRIDE, FINAL.

Following the Google/Chromium coding style wrt. virtual, OVERRIDE and
FINAL specifications.

TEST=unittests
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/816453005

Cr-Commit-Position: refs/heads/master@{#25924}