Previously WebrtcTransport clients had to use incoming_channel_factory() to
accept incoming data channels. That worked only if the receiver knows in
advance names of all channels it can receive. Now the transport calls
EventHandler for incoming data channels, which allows the receiver to decide
dynamically if it wants to accept that channel. Also channels now can be
closed dynamically and the transport doesn't terminate connection when one
of the channels is closed.

BUG=621691

Review-Url: https://codereview.chromium.org/2146213002
Cr-Commit-Position: refs/heads/master@{#406639}