virtual/override specifier cleanup in content/.

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

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

R=nasko@chromium.org
BUG=417463

Review URL: https://codereview.chromium.org/794333003

Cr-Commit-Position: refs/heads/master@{#309997}
8 files changed