Update {virtual,override} to follow C++11 style in extensions.

Update {virtual,override} to follow C++11 style in extensions.

The Google style guide states that only one of {virtual,override,final} should be used for each declaration, since override implies virtual and final implies both virtual and override.

This patch was manually generated using a regex and a text editor.

BUG=417463
Committed: https://crrev.com/0ec9f9fbbc048a0a05b74cad9c6ee795f89bb641
Cr-Commit-Position: refs/heads/master@{#326545}

[ncarter (slow), 2015-04-23 02:11:06]

nick@chromium.org changed reviewers:
+ rdevlin.cronin@chromium.org

[Devlin, 2015-04-23 15:28:12]

lgtm

[ncarter (slow), 2015-04-23 16:37:14]

The CQ bit was checked by nick@chromium.org

[commit-bot: I haz the power, 2015-04-23 16:38:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1104543004/1

[commit-bot: I haz the power, 2015-04-23 16:43:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-23 16:44:24]

Patchset 1 (id:??) landed as
https://crrev.com/0ec9f9fbbc048a0a05b74cad9c6ee795f89bb641
Cr-Commit-Position: refs/heads/master@{#326545}