Add offer_extmap_allow_mixed to RTCConfiguration

Bug: webrtc:9986
Change-Id: I346e03a46f35c7d59d3ae769842e3aeec9d2d50d
Reviewed-on: https://webrtc-review.googlesource.com/c/110501
Commit-Queue: Johannes Kron <kron@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25596}
diff --git a/api/peerconnectioninterface.h b/api/peerconnectioninterface.h
index 80c3091..044a480 100644
--- a/api/peerconnectioninterface.h
+++ b/api/peerconnectioninterface.h
@@ -597,6 +597,14 @@
     // settings set in PeerConnectionFactory (which is deprecated).
     absl::optional<CryptoOptions> crypto_options;
 
+    // Configure if we should include the SDP attribute extmap-allow-mixed in
+    // our offer. Although we currently do support this, it's not included in
+    // our offer by default due to a previous bug that caused the SDP parser to
+    // abort parsing if this attribute was present. This is fixed in Chrome 71.
+    // TODO(webrtc:9985): Change default to true once sufficient time has
+    // passed.
+    bool offer_extmap_allow_mixed = false;
+
     //
     // Don't forget to update operator== if adding something.
     //