Allow transceivers to be not found in more cases.

This fixes the case where a media section is rejected in an answer,
something that is done by SFUs, but not possible using transceiver.stop().

Bug: chromium:1134686
Change-Id: Ia33579070093ab70c4191710fd1dcb3ca377befd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/187349
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32363}

pc/sdp_offer_answer.cc

diff --git a/pc/sdp_offer_answer.cc b/pc/sdp_offer_answer.cc
index 5dc5571..c6432a6 100644
--- a/pc/sdp_offer_answer.cc
+++ b/pc/sdp_offer_answer.cc
@@ -2871,17 +2871,16 @@
         old_remote_content =
             &old_remote_description->description()->contents()[i];
       }
-      // In the case where an m-section has completed its rejection,
-      // and is not being reused, we do not expect a transceiver.
-      if (old_local_content && old_local_content->rejected &&
-          old_remote_content && old_remote_content->rejected &&
-          new_content.rejected) {
-        continue;
-      }
       auto transceiver_or_error =
           AssociateTransceiver(source, new_session.GetType(), i, new_content,
                                old_local_content, old_remote_content);
       if (!transceiver_or_error.ok()) {
+        // In the case where a transceiver is rejected locally, we don't
+        // expect to find a transceiver, but might find it in the case
+        // where state is still "stopping", not "stopped".
+        if (new_content.rejected) {
+          continue;
+        }
         return transceiver_or_error.MoveError();
       }
       auto transceiver = transceiver_or_error.MoveValue();
@@ -2946,8 +2945,9 @@
       transceiver = transceivers().FindByMLineIndex(mline_index);
     }
     if (!transceiver) {
+      // This may happen normally when media sections are rejected.
       LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
-                           "Unknown transceiver");
+                           "Transceiver not found based on m-line index");
     }
   } else {
     RTC_DCHECK_EQ(source, cricket::CS_REMOTE);