Add more implementations of pure virtuals

These methods are being made pure virtual in WebRTC API.

Bug: webrtc:11839
Change-Id: Ib3a4287cc34fd24dc2fdde0926ef515568431b86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3386935


Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#959307}