[1/2] - Make new version pure-virtual, and deprecated version non-pure.
This will allow deleting the deprecated version from downstream
projects.
[2/2] - Remove deprecated version.
TBR=sprang@webrtc.org,stefan@webrtc.org
Bug: webrtc:10336
Change-Id: I3904da12ec471980adfb22f2e61304d42de4ec66
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144043
Reviewed-by: Elad Alon <eladalon@webrtc.org>
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28419}