The src/remoting/protocol/channel_socket_adapter_unittest.cc will use this class
instead of creating its own MockTransportChannel which subclasses the TransportChannel.
This will make it easier to switch the base class of P2PTransportChannel from
TransportChannel to IceTransportInternal.
BUG=webrtc:6951
Review-Url: https://codereview.webrtc.org/2610663003
Cr-Commit-Position: refs/heads/master@{#15922}