Refactoring that removes P2PTransport and DtlsTransport classes.

Their base class, Transport, still exists, but it now has a more specific
role: a helper class that applies TransportDescriptions. And is renamed
to JsepTransport as a result.

TransportController is now the entity primarily responsible for managing
TransportChannels. It also starts storing pointers to the DTLS and ICE
chanels separately, which will make it easier to remove
TransportChannel/TransportChannelImpl in a subsequent CL.

BUG=None

Review-Url: https://codereview.webrtc.org/2517883002
Cr-Original-Commit-Position: refs/heads/master@{#15450}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: bd28681d02dee8c185aeb39207e8154f0ad14a37
27 files changed