Standardize usage of virtual/override/final in extensions/

Standardize usage of virtual/override/final in extensions/

This patch was automatically generated by applying clang fixit hints
generated by the plugin to the source tree.

BUG=417463
TBR=yoz@chromium.org

Committed: https://crrev.com/9168b2fe3e30afa4fd9a9500ba0725edf97c20a3
Cr-Commit-Position: refs/heads/master@{#300475}

[dcheng, 2014-10-21 09:50:04]

The CQ bit was checked by dcheng@chromium.org

[commit-bot: I haz the power, 2014-10-21 09:51:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/664933004/1

[commit-bot: I haz the power, 2014-10-21 11:15:45]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-10-21 11:15:47]

Try jobs failed on following builders:
  linux_chromium_rel_swarming on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[dcheng, 2014-10-21 11:16:11]

The CQ bit was checked by dcheng@chromium.org

[commit-bot: I haz the power, 2014-10-21 11:19:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/664933004/1

[commit-bot: I haz the power, 2014-10-21 12:38:34]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-21 13:10:08]

Patchset 1 (id:??) landed as
https://crrev.com/9168b2fe3e30afa4fd9a9500ba0725edf97c20a3
Cr-Commit-Position: refs/heads/master@{#300475}