Reland "Implement rollback for setRemoteDescription"

This is a reland of 16d4c4d4fbb8644033def1091d2d5c941c1b01fa after
downstream project was updated to be prepared for the new SdpType.

Original change's description:
> Implement rollback for setRemoteDescription
>
> Bug: chromium:980875
> Change-Id: I4575e9ad1902a20937f9812f49edee2a2441f76d
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/153525
> Commit-Queue: Eldar Rello <elrello@microsoft.com>
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Reviewed-by: Henrik Boström <hbos@webrtc.org>
> Reviewed-by: Steve Anton <steveanton@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#29422}

TBR=steveanton@webrtc.org

Bug: chromium:980875
Change-Id: Iba8d25bf2dc481b25a03eeae9818bd5f4c3eaa2d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/156569
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29460}

api/peer_connection_interface.h

@@ -659,6 +659,9 @@ class RTC_EXPORT PeerConnectionInterface : public rtc::RefCountInterface {
     // logs with TURN server logs. It will not be added if it's an empty string.
     std::string turn_logging_id;
 
+    // Added to be able to control rollout of this feature.
+    bool enable_implicit_rollback = false;
+
     //
     // Don't forget to update operator== if adding something.
     //