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/ARDWebSocketChannel.m b/webrtc/examples/objc/AppRTCDemo/ARDWebSocketChannel.m
index 87a690e..6f60380 100644
--- a/webrtc/examples/objc/AppRTCDemo/ARDWebSocketChannel.m
+++ b/webrtc/examples/objc/AppRTCDemo/ARDWebSocketChannel.m
@@ -233,6 +233,7 @@
       // Should not receive answer in loopback scenario.
       break;
     case kARDSignalingMessageTypeCandidate:
+    case kARDSignalingMessageTypeCandidateRemoval:
       // Send back to server.
       [self sendMessage:message];
       break;