Cleanup redundant "virtual+override" in mockpeerconnection.

Update {virtual,override} to follow C++11 style in content/renderer/media.

This CL has just two files which got some extra "while I'm in there" cleanup while doing mechanical fixups across the codebase.

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.

BUG=417463
Committed: https://crrev.com/af05b2a50818299ef27c8fc84fd2f65b37401329
Cr-Commit-Position: refs/heads/master@{#326972}

[ncarter (slow), 2015-04-24 19:52:17]

nick@chromium.org changed reviewers:
+ tommi@chromium.org

[ncarter (slow), 2015-04-24 19:52:17]

tommi: Please review! Thanks

[tommi (sloooow) - chröme, 2015-04-25 15:28:35]

The CQ bit was checked by tommi@chromium.org

[tommi (sloooow) - chröme, 2015-04-25 15:28:36]

lgtm

[commit-bot: I haz the power, 2015-04-25 15:28:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1106043002/20001

[commit-bot: I haz the power, 2015-04-25 15:40:33]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-25 15:41:37]

Patchset 2 (id:??) landed as
https://crrev.com/af05b2a50818299ef27c8fc84fd2f65b37401329
Cr-Commit-Position: refs/heads/master@{#326972}