After this change, SdpOfferAnswerHandler implements a read-only
interface called SdpStateProvider, which allows enough access
for WebRtcSessionDescriptionFactory to learn what it needs to know.
Bug: webrtc:12060
Change-Id: Ic888b5027b2df5fee407d32b89da66ff044c40de
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/190145
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32486}