Pass around the candidate removals events in IOS clients

When local candidates are removed, signal to RTCPeerConnection
and eventually send to the remote client.
When a candidate-removal message is received, notify the native PeerConnection.

BUG=
R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1972483002 .

Cr-Commit-Position: refs/heads/master@{#12852}
diff --git a/webrtc/examples/objc/AppRTCDemo/RTCICECandidate+JSON.h b/webrtc/examples/objc/AppRTCDemo/RTCICECandidate+JSON.h
index 1051f8e..d2e5e33 100644
--- a/webrtc/examples/objc/AppRTCDemo/RTCICECandidate+JSON.h
+++ b/webrtc/examples/objc/AppRTCDemo/RTCICECandidate+JSON.h
@@ -13,6 +13,10 @@
 @interface RTCIceCandidate (JSON)
 
 + (RTCIceCandidate *)candidateFromJSONDictionary:(NSDictionary *)dictionary;
++ (NSArray<RTCIceCandidate *> *)candidatesFromJSONDictionary:
+    (NSDictionary *)dictionary;
++ (NSData *)JSONDataForIceCandidates:(NSArray<RTCIceCandidate *> *)candidates
+                            withType:(NSString *)typeValue;
 - (NSData *)JSONData;
 
 @end