virtual/override specifier cleanup in media/.

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

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

R=xhwang@chromium.org
BUG=417463

Review URL: https://codereview.chromium.org/829413002

Cr-Commit-Position: refs/heads/master@{#309979}
3 files changed