virtual/override specifier cleanup in ui/ (mostly Ozone code).

These are changes detected by Chromecast not covered by the Linux build.

See: https://codereview.chromium.org/831863003

R=sky@chromium.org,spang@chromium.org
BUG=417463

Review URL: https://codereview.chromium.org/808333004

Cr-Commit-Position: refs/heads/master@{#310019}
13 files changed