This consolidates termination logic in the DataChannelController
to make shut down consistent between when the transport notifies
of termination and when termination is initiated from the PC side.
This removes the need for `OnTransportChannelClosed` from the PC
side since we can just use TeardownDataChannelTransport_n (the two
were always being called together).
Bug: webrtc:11547
Change-Id: I1763f82cbfe1a3d5b8bfabb8d4cba0ee0fa95738
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/300561
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39838}
b00d63c88b