QuicDataChannel implements DataChannelInterface. It
replaces SCTP data channels by using a QuicTransportChannel
to create a ReliableQuicStream for each message.
QuicDataChannel only implements unordered, reliable delivery
for the initial implementation and does not send a hello message.
QuicDataTransport is a helper class that dispatches each incoming
ReliableQuicStream to a QuicDataChannel when the remote
peer receives a message by parsing the data channel id and message id
from the message header. It is also responsible for encoding the header
before QuicDataChannel sends the message.
Split from CL https://codereview.chromium.org/1844803002/.
BUG=
Review-Url: https://codereview.webrtc.org/1886623002
Cr-Commit-Position: refs/heads/master@{#12574}
