Signal extmap-allow-mixed by default on session level

The extmap-allow-mixed SDP attribute signals that one- and two-byte RTP
header extensions can be mixed. In practice, this also means that WebRTC
will support two-byte RTP header extensions when this is signaled by
both peers.

Bug: webrtc:9985
Change-Id: I80a3f97bab162c7d9a5acf2cae07b977641c039d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/197943
Commit-Queue: Emil Lundmark <lndmrk@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33036}
diff --git a/api/peer_connection_interface.h b/api/peer_connection_interface.h
index 92d965b..d7f4e19f 100644
--- a/api/peer_connection_interface.h
+++ b/api/peer_connection_interface.h
@@ -621,12 +621,8 @@
     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;
+    // our offer on session level.
+    bool offer_extmap_allow_mixed = true;
 
     // TURN logging identifier.
     // This identifier is added to a TURN allocation