Adding Java and Obj-C bindings for RtpEncodingParameters.ssrc.

BUG=webrtc:6903

Review-Url: https://codereview.webrtc.org/2581913002
Cr-Commit-Position: refs/heads/master@{#15936}

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

@@ -25,6 +25,9 @@ RTC_EXPORT
  */
 @property(nonatomic, copy, nullable) NSNumber *maxBitrateBps;
 
+/** The SSRC being used by this encoding. */
+@property(nonatomic, readonly, nullable) NSNumber *ssrc;
+
 - (instancetype)init NS_DESIGNATED_INITIALIZER;
 
 @end