Renamed dtlstransportchannel.h/.cc/_unittest.cc.

Renamed the dtlstransportchannel.h/.cc to dtlstransport.h/.cc which are
the right names for those files because the concept of DtlsTransportChannel
no longer exists.

BUG=None

Review-Url: https://codereview.webrtc.org/3004503002
Cr-Original-Commit-Position: refs/heads/master@{#19478}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: ca6d3b60b521184a9c641162cf3dfad2dd0ff9e9
7 files changed