Onion Soup DataChannel.

Merge the DataChannel implementation from content/ into blink/.

Remove content::RtcDataChannelHandler and the interfaces between content/ and
blink/, WebRTCDataChannelHandler and WebRTCDataChannelHandlerClient. Move
content::RtcDataChannelHandler::Observer into blink::RTCDataChannel::Observer.
Have blink::RTCDataChannel register an webrtc::DataChannel observer instead of
content::RtcDataChannelHandler. When creating data channels in peer connection
implementation, pass on the webrtc::DataChannelInterface to
blink::RTCDataChannel instead of creating a content::RtcDataChannelHandler.

Design doc: http://doc/18SC6XjEoe236DGkxtVLuojZ4lqb8i8soCsrJixOEiR8

Bug: 931698
Change-Id: I5eb773fe183ac2a4490eea6c7a9b911d210b363a
Reviewed-on: https://chromium-review.googlesource.com/c/1475468
Commit-Queue: Marina Ciocea <marinaciocea@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637129}
22 files changed