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}

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.
     //