Update checklicenses.py to allow GPL v3 in ffmpeg code

Update checklicenses.py to allow GPL v3 in ffmpeg code

BUG=

Committed: https://crrev.com/a2b9474fa2a7bccd0a353922967e9974146b2144
Cr-Commit-Position: refs/heads/master@{#296962}

[jrummell, 2014-09-25 22:06:15]

jrummell@chromium.org changed reviewers:
+ dalecurtis@chromium.org, phajdan.jr@chromium.org

[jrummell, 2014-09-25 22:06:16]

PTAL. The offending ffmpeg file was removed so that the ffmpeg roll could get in
well before the branch cut. However, the next roll will hit the same issue, so
updating the script so it doesn't happen again.

[Paweł Hajdan Jr., 2014-09-26 00:38:27]

LGTM

[DaleCurtis, 2014-09-26 01:07:14]

lgtm

[jrummell, 2014-09-26 16:19:12]

The CQ bit was checked by jrummell@chromium.org

[commit-bot: I haz the power, 2014-09-26 16:20:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/607693002/1

[commit-bot: I haz the power, 2014-09-26 16:59:03]

Committed patchset #1 (id:1) as 4e9357b134785a7fa4b2fa67deea6bc5e7ea3f15

[commit-bot: I haz the power, 2014-09-26 16:59:48]

Patchset 1 (id:??) landed as
https://crrev.com/a2b9474fa2a7bccd0a353922967e9974146b2144
Cr-Commit-Position: refs/heads/master@{#296962}