Add support for JSEP offer/answer with transceivers

This change adds support to PeerConnection's CreateOffer/
CreateAnswer/SetLocalDescription/SetRemoteDescription for
Unified Plan SDP mapping to/from RtpTransceivers. This behavior
is enabled using the kUnifiedPlan SDP semantics in the
PeerConnection configuration.

Bug: webrtc:7600
Change-Id: I4b44f5d3690887d387bf9c47eac00db8ec974571
Reviewed-on: https://webrtc-review.googlesource.com/28341
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Peter Thatcher <pthatcher@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21442}

pc/peerconnectionwrapper.cc

diff --git a/pc/peerconnectionwrapper.cc b/pc/peerconnectionwrapper.cc
index 121cc64..d297054 100644
--- a/pc/peerconnectionwrapper.cc
+++ b/pc/peerconnectionwrapper.cc
@@ -179,6 +179,45 @@
   return observer->result();
 }
 
+bool PeerConnectionWrapper::ExchangeOfferAnswerWith(
+    PeerConnectionWrapper* answerer) {
+  RTC_DCHECK(answerer);
+  if (answerer == this) {
+    RTC_LOG(LS_ERROR) << "Cannot exchange offer/answer with ourself!";
+    return false;
+  }
+  auto offer = CreateOffer();
+  EXPECT_TRUE(offer);
+  if (!offer) {
+    return false;
+  }
+  bool set_local_offer =
+      SetLocalDescription(CloneSessionDescription(offer.get()));
+  EXPECT_TRUE(set_local_offer);
+  if (!set_local_offer) {
+    return false;
+  }
+  bool set_remote_offer = answerer->SetRemoteDescription(std::move(offer));
+  EXPECT_TRUE(set_remote_offer);
+  if (!set_remote_offer) {
+    return false;
+  }
+  auto answer = answerer->CreateAnswer();
+  EXPECT_TRUE(answer);
+  if (!answer) {
+    return false;
+  }
+  bool set_local_answer =
+      answerer->SetLocalDescription(CloneSessionDescription(answer.get()));
+  EXPECT_TRUE(set_local_answer);
+  if (!set_local_answer) {
+    return false;
+  }
+  bool set_remote_answer = SetRemoteDescription(std::move(answer));
+  EXPECT_TRUE(set_remote_answer);
+  return set_remote_answer;
+}
+
 rtc::scoped_refptr<RtpTransceiverInterface>
 PeerConnectionWrapper::AddTransceiver(cricket::MediaType media_type) {
   RTCErrorOr<rtc::scoped_refptr<RtpTransceiverInterface>> result =