Add temporal-layers option to video_loopback.

Add temporal-layers option to video_loopback.

BUG=
R=asapersson@webrtc.org, sprang@webrtc.org

Committed: https://crrev.com/9874ee0d7a3c2ae6000ae6e82ae9087986b0469e
Cr-Commit-Position: refs/heads/master@{#9482}

[pbos-webrtc, 2015-06-17 09:48:17]

PTAL, asapersson@ for the feature and sprang@ to verify that I didn't destroy
screenshare loopback.

[pbos-webrtc, 2015-06-17 09:48:26]

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

[commit-bot: I haz the power, 2015-06-17 09:48:37]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1194533002/1

[commit-bot: I haz the power, 2015-06-17 11:00:06]

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

[commit-bot: I haz the power, 2015-06-17 11:00:07]

Dry run: This issue passed the CQ dry run.

[åsapersson, 2015-06-22 09:19:26]

On 2015/06/17 11:00:07, commit-bot: I haz the power wrote:
> Dry run: This issue passed the CQ dry run.

lgtm

[pbos-webrtc, 2015-06-22 09:35:54]

The CQ bit was checked by pbos@webrtc.org

[commit-bot: I haz the power, 2015-06-22 09:36:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1194533002/1

[pbos-webrtc, 2015-06-22 09:39:05]

The CQ bit was unchecked by pbos@webrtc.org

[pbos-webrtc, 2015-06-22 09:39:24]

On 2015/06/22 09:36:02, commit-bot: I haz the power wrote:
> CQ is trying da patch. Follow status at
>  https://chromium-cq-status.appspot.com/patch-status/1194533002/1

Oops, CQ aborted. sprang@ ptal and then check the commit bit :)

[sprang_webrtc, 2015-06-22 11:41:25]

lgtm

[pbos-webrtc, 2015-06-22 11:43:16]

The CQ bit was checked by pbos@webrtc.org

[commit-bot: I haz the power, 2015-06-22 11:43:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1194533002/1

[commit-bot: I haz the power, 2015-06-22 11:44:30]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-22 11:44:40]

Patchset 1 (id:??) landed as
https://crrev.com/9874ee0d7a3c2ae6000ae6e82ae9087986b0469e
Cr-Commit-Position: refs/heads/master@{#9482}