Use rtc::PacketTransportInterface in WebrtcSession unit test.
Use rtc::PacketTransportInterface in WebrtcSession unit test.

Using rtc::PacketTransportInterface instead of cricket::TransportChannel
is a preparation for refactoring channel.cc.

BUG=webrtc:6676

Review-Url: https://codereview.webrtc.org/2483093003
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14989}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 669d69bd1512e8613acc9b6d520e575ac78e9ae0

Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk