Add pruneTurnPorts to the IOS RTCConfiguration.

BUG=
R=pthatcher@webrtc.org
TBR=tkchin@webrt.org

Review URL: https://codereview.webrtc.org/2120553002 .

Cr-Commit-Position: refs/heads/master@{#13368}

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCConfiguration.h

@@ -93,6 +93,9 @@ RTC_EXPORT
 /** ICE candidate pool size as defined in JSEP. Default is 0. */
 @property(nonatomic, assign) int iceCandidatePoolSize;
 
+/** Prune turn ports on the same network to the same turn server. Default is false. */
+@property(nonatomic, assign) bool pruneTurnPorts;
+
 /** If set to true, this means the ICE transport should presume TURN-to-TURN
  *  candidate pairs will succeed, even before a binding response is received.
  */