Add setSinkId method to WebMediaPlayer.

Add setSinkId method to WebMediaPlayer.

This is inteded to support the Audio Output Devices API
(http://w3c.github.io/mediacapture-output/)
https://www.chromestatus.com/feature/4621603249848320

BUG=438023
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197215

[Guido Urdaneta, 2015-06-16 10:47:03]

guidou@chromium.org changed reviewers:
+ mkwst@chromium.org, tommi@chromium.org

[Guido Urdaneta, 2015-06-16 10:48:43]

guidou@chromium.org changed reviewers:
+ wolenetz@google.com

[Guido Urdaneta, 2015-06-16 10:50:06]

Hi, please review.

wolenetz: As agreed, this includes the comment about threading and the
WebCallbacks object. Please let me know if you have any comments.

[tommi (sloooow) - chröme, 2015-06-16 14:45:24]

lgtm

[Guido Urdaneta, 2015-06-16 15:43:32]

guidou@chromium.org changed reviewers:
+ japhet@chromium.org

[Guido Urdaneta, 2015-06-16 23:33:46]

guidou@chromium.org changed reviewers:
+ tkent@chromium.org
- japhet@chromium.org

[tkent, 2015-06-17 00:36:52]

lgtm

[Guido Urdaneta, 2015-06-17 00:37:56]

The CQ bit was checked by guidou@chromium.org

[commit-bot: I haz the power, 2015-06-17 00:39:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1188713004/1

[commit-bot: I haz the power, 2015-06-17 00:43:43]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197215