Allow passing null for RTCcodecSpecific info in encoder callback.

Allow passing null for RTCcodecSpecific info in encoder callback.

BUG=None

[kthelgason, 2017-09-12 14:42:32]

The CQ bit was checked by kthelgason@webrtc.org to run a CQ dry run

[commit-bot: I haz the power, 2017-09-12 14:42:43]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.webrtc.org/...

[commit-bot: I haz the power, 2017-09-12 14:52:12]

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

[commit-bot: I haz the power, 2017-09-12 14:52:13]

Dry run: Try jobs failed on following builders:
  ios_api_framework on master.tryserver.webrtc (JOB_FAILED,
http://build.chromium.org/p/tryserver.webrtc/builders/ios_api_framework/build...)

[kthelgason, 2017-09-13 07:05:49]

The CQ bit was checked by kthelgason@webrtc.org to run a CQ dry run

[commit-bot: I haz the power, 2017-09-13 07:05:53]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.webrtc.org/...

[kthelgason, 2017-09-13 07:24:29]

kthelgason@webrtc.org changed reviewers:
+ magjed@webrtc.org

[kthelgason, 2017-09-13 07:24:30]

Make sure to be sitting down for this one!

[commit-bot: I haz the power, 2017-09-13 07:30:43]

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

[commit-bot: I haz the power, 2017-09-13 07:30:44]

Dry run: This issue passed the CQ dry run.

[magjed_webrtc, 2017-09-14 10:25:09]

lgtm :)

[kthelgason, 2017-09-28 13:47:58]

The CQ bit was checked by kthelgason@webrtc.org

[commit-bot: I haz the power, 2017-09-28 13:48:04]

CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.webrtc.org/...

[commit-bot: I haz the power, 2017-09-28 13:52:33]

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

[commit-bot: I haz the power, 2017-09-28 13:52:34]

Try jobs failed on following builders:
  mac_baremetal on master.tryserver.webrtc (JOB_FAILED,
http://build.chromium.org/p/tryserver.webrtc/builders/mac_baremetal/builds/25373)