Expose adaptive_ptime from iOS SDK.

Bug: None
Change-Id: I48fd0937f51dc972b3eccd66f99ae80378e32fe1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/214968
Commit-Queue: Yura Yaroshevich <yura.yaroshevich@gmail.com>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33766}

sdk/objc/api/peerconnection/RTCRtpEncodingParameters.h

@@ -65,6 +65,10 @@ RTC_OBJC_EXPORT
 /** The relative DiffServ Code Point priority. */
 @property(nonatomic, assign) RTCPriority networkPriority;
 
+/** Allow dynamic frame length changes for audio:
+ https://w3c.github.io/webrtc-extensions/#dom-rtcrtpencodingparameters-adaptiveptime */
+@property(nonatomic, assign) BOOL adaptiveAudioPacketTime;
+
 - (instancetype)init;
 
 @end