Update {virtual,override,final} to follow C++11 style in chrome/browser/chromeos...

Update {virtual,override,final} to follow C++11 style in chrome/browser/chromeos/policy.

The Google style guide states that only one of {virtual,override,final}
should be used for each declaration, since override implies virtual and
final implies both virtual and override.

This patch was automatically generated with a ChromeOS build using a
variation of https://codereview.chromium.org/598073004.

BUG=417463
R=atwilson@chromium.org

Review URL: https://codereview.chromium.org/835673004

Cr-Commit-Position: refs/heads/master@{#311871}