virtual/override specifier cleanup in device/battery/.

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

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

R=timvolodine@chromium.org
BUG=417463

Review URL: https://codereview.chromium.org/822683005

Cr-Commit-Position: refs/heads/master@{#310098}
1 file changed