Add PeerConnectionObserver::OnRemoveTrack callback.

Add PeerConnectionObserver::OnRemoveTrack callback.

BUG=None

[korniltsev, 2017-07-25 22:27:10]

Description was changed from

==========
Add PeerConnectionObserver::OnRemoveTrack callback.

BUG=None
==========

to

==========
Add PeerConnectionObserver::OnRemoveTrack callback.

BUG=None
==========

[korniltsev, 2017-07-25 22:27:10]

korniltsev.anatoly@gmail.com changed reviewers:
+ deadbeef@webrtc.org

[korniltsev, 2017-07-25 22:29:20]

It looks like there is no way to find that the remote track is removed. This
patch adds missing callback.

[Taylor Brandstetter, 2017-07-25 23:19:46]

Since there isn't a direct analog to this in the spec, and this is still being
actively discussed (see this issue I filed:
https://github.com/w3c/webrtc-pc/issues/1161), I'd prefer holding off on this
until things stabilize.

In the meantime, there *is* an existing way to tell when this happens, as long
as the track is added to a stream (which it always is, currently).
MediaStreamInterface inherits from NotifierInterface, meaning you can add an
observer that receives callbacks when it changes. A track being removed will
trigger this callback. I even wrote a helper class that turns these generic
callbacks into "track added"/"track removed" signals:
https://cs.chromium.org/chromium/src/third_party/webrtc/pc/mediastreamobserver.h

Would that work for you for the time being?

[korniltsev, 2017-07-26 07:26:58]

Thank you for suggestion. This will work.
Just to clarify. I need this events in jni and objc, so I can stop/start
rendering remote track when it is removed/added.
If I implement OnRemoveTrack callback in jni and objc with a
MediaStreamObserver, will you be able to review and merge it to upstream or we
should wait for spec approval anyway?  
Thanks.

[Taylor Brandstetter, 2017-07-26 18:02:55]

On 2017/07/26 07:26:58, korniltsev wrote:
> Thank you for suggestion. This will work.
> Just to clarify. I need this events in jni and objc, so I can stop/start
> rendering remote track when it is removed/added.
> If I implement OnRemoveTrack callback in jni and objc with a
> MediaStreamObserver, will you be able to review and merge it to upstream or we
> should wait for spec approval anyway?  
> Thanks.

The Java and Objective-C "MediaStream" objects definitely *should* be kept in
sync with the C++ objects, meaning if a track is removed from a C++ MediaStream
it should disappear from the Java MediaStream as well. And there should be a
similar observer interface for the application to listen for modifications.

It's been something I've wanted to fix for a long time, but it hasn't been a top
priority so I haven't had a chance to do it. If you want to do this, go ahead,
I'd be grateful.

Here's a bug to reference:
https://bugs.chromium.org/p/webrtc/issues/detail?id=5677