Introduing IceTransportInternal2 is a temporary fix to switch the base class of P2PTransportChannel to IceTransportInternal without breaking Chromium. It is removed in this CL. Reland this CL after Chromium doesn't depend on this. BUG=webrtc:6951 Review-Url: https://codereview.webrtc.org/2632563002 Cr-Commit-Position: refs/heads/master@{#16131}