Propagate SDP negotiation of extmap-allow-mixed to RtpHeaderExtensionMap

Bug: webrtc:7990
Change-Id: I662595f90b9d0be39f7e14752e13b2bb7a1746ee
Reviewed-on: https://webrtc-review.googlesource.com/c/106020
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25421}

modules/rtp_rtcp/source/rtp_rtcp_impl.h

@@ -70,6 +70,8 @@ class ModuleRtpRtcpImpl : public RtpRtcp, public RTCPReceiver::ModuleRtpRtcp {
 
   int32_t DeRegisterSendPayload(int8_t payload_type) override;
 
+  void SetExtmapAllowMixed(bool extmap_allow_mixed) override;
+
   // Register RTP header extension.
   int32_t RegisterSendRtpHeaderExtension(RTPExtensionType type,
                                          uint8_t id) override;