DataChannelInterface: Remove default implementation of methods.
DataChannelInterface: Remove default implementation of methods.

This can land as soon as https://codereview.chromium.org/2520033005/ has
landed.

BUG=chromium:654927

Review-Url: https://codereview.webrtc.org/2524033002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#15221}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: c42d37646f8e28a54e628cc68c6c3c3c0bfa68f4

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