Expose offerExtmapAllowMixed in iOS SDK.

Bug: None
Change-Id: Ic14d1f005b6c727b509492399901d822bd6950db
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/212280
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33498}

sdk/objc/api/peerconnection/RTCConfiguration.h

@@ -227,6 +227,12 @@ RTC_OBJC_EXPORT
  */
 @property(nonatomic, assign) BOOL enableImplicitRollback;
 
+/**
+ * Control if "a=extmap-allow-mixed" is included in the offer.
+ * See: https://www.chromestatus.com/feature/6269234631933952
+ */
+@property(nonatomic, assign) BOOL offerExtmapAllowMixed;
+
 - (instancetype)init;
 
 @end