Add a few missing overrides found by a new clang warning.

Add a few missing overrides found by a new clang warning.

Namely, -Winconsistent-missing-override. While here, also remove "virtual"
from the functions I'm marking override, like our new override style dictates.
No behavior change.

BUG=428099, 417463
Committed: https://crrev.com/ef3e4e6cff826927f51cfc74c6d84196e663dfeb
Cr-Commit-Position: refs/heads/master@{#302018}

[Nico, 2014-10-30 01:14:19]

thakis@chromium.org changed reviewers:
+ thestig@chromium.org

[Lei Zhang, 2014-10-30 01:24:59]

lgtm

[Nico, 2014-10-30 01:26:10]

The CQ bit was checked by thakis@chromium.org

[commit-bot: I haz the power, 2014-10-30 01:26:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/690613005/1

[commit-bot: I haz the power, 2014-10-30 02:10:13]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-30 02:10:57]

Patchset 1 (id:??) landed as
https://crrev.com/ef3e4e6cff826927f51cfc74c6d84196e663dfeb
Cr-Commit-Position: refs/heads/master@{#302018}