Make protocol::ConnectionToClient an abstract interface.

Moved existing code to IceConnectionToClient. In future a new
WebrtcConnectionToClient will handle webrtc-based connection.

BUG=547158

Review URL: https://codereview.chromium.org/1460593005

Cr-Commit-Position: refs/heads/master@{#360673}
13 files changed