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-Original-Commit-Position: refs/heads/master@{#16131}
Committed: c7953fa716
Review-Url: https://codereview.webrtc.org/2632563002
Cr-Commit-Position: refs/heads/master@{#16960}