[Media Router] Set initial sink availability to available in MRMojoImpl.

[Media Router] Set initial sink availability to available in MRMojoImpl.

To be submitted after MR extension has picked up the change to send
SinkAvailability updates.

BUG=530702
Committed: https://crrev.com/62e8f2fa305568f855fb2e30834c20fab4891f1a
Cr-Commit-Position: refs/heads/master@{#358748}

[imcheng, 2015-11-03 01:17:02]

Patchset #1 (id:1) has been deleted

[imcheng, 2015-11-04 02:01:07]

imcheng@chromium.org changed reviewers:
+ mfoltz@chromium.org

[imcheng, 2015-11-04 02:01:07]

Mark: PTAL

[mark a. foltz, 2015-11-09 18:35:31]

LGTM

Please test with the current stable extension before submitting so as not to
break current MR users :)

[imcheng, 2015-11-10 00:29:21]

Tested on stable extension. Everything works except for one case (and it has to
do with the original SinkAvailability logic that I overlooked, rather than this
patch): when registering an observer while SinkAvailability is UNAVAILABLE, the
observer never gets a call back. In practice, it means
PresentationRequest.getAvailability() never resolves. This will require a
separate fix - the register-on-construction logic in MediaSinksObserver
currently prevents us from returning an empty sink list synchronously.

[imcheng, 2015-11-10 00:30:01]

The CQ bit was checked by imcheng@chromium.org

[commit-bot: I haz the power, 2015-11-10 00:36:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1420533008/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1420533008/20001

[commit-bot: I haz the power, 2015-11-10 02:01:14]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-11-10 02:02:14]

Patchset 1 (id:??) landed as
https://crrev.com/62e8f2fa305568f855fb2e30834c20fab4891f1a
Cr-Commit-Position: refs/heads/master@{#358748}