Fix VP9 support in AppRTCDemo.

Fix VP9 support in AppRTCDemo.

Default VP9 selection is no longer triggered by field trial string
after https://codereview.webrtc.org/1432673002, so VP9 need to
be selected now through SDP mangling.

R=wzh@webrtc.org

Committed: https://crrev.com/eb8b388273d14ed83495801e9e4e5de12bb00ad1
Cr-Commit-Position: refs/heads/master@{#10660}

[AlexG, 2015-11-16 20:07:15]

glaznev@webrtc.org changed reviewers:
+ wzh@webrtc.org

[AlexG, 2015-11-16 20:07:16]

PTAL

[AlexG, 2015-11-16 20:09:31]

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

[commit-bot: I haz the power, 2015-11-16 20:09:34]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1452783002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1452783002/1

[commit-bot: I haz the power, 2015-11-16 21:26:17]

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

[commit-bot: I haz the power, 2015-11-16 21:26:18]

Dry run: This issue passed the CQ dry run.

[wzh, 2015-11-16 21:57:25]

lgtm

[AlexG, 2015-11-16 22:12:08]

Committed patchset #1 (id:1) manually as
eb8b388273d14ed83495801e9e4e5de12bb00ad1 (presubmit successful).

[commit-bot: I haz the power, 2015-11-16 22:12:12]

Patchset 1 (id:??) landed as
https://crrev.com/eb8b388273d14ed83495801e9e4e5de12bb00ad1
Cr-Commit-Position: refs/heads/master@{#10660}