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}

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

@@ -98,6 +98,10 @@ RTC_EXPORT
 - (void)peerConnection:(RTCPeerConnection *)peerConnection
     didGenerateIceCandidate:(RTCIceCandidate *)candidate;
 
+/** Called when a group of local Ice candidates have been removed. */
+- (void)peerConnection:(RTCPeerConnection *)peerConnection
+    didRemoveIceCandidates:(NSArray<RTCIceCandidate *> *)candidates;
+
 /** New data channel has been opened. */
 - (void)peerConnection:(RTCPeerConnection *)peerConnection
     didOpenDataChannel:(RTCDataChannel *)dataChannel;
@@ -148,6 +152,9 @@ RTC_EXPORT
 /** Provide a remote candidate to the ICE Agent. */
 - (void)addIceCandidate:(RTCIceCandidate *)candidate;
 
+/** Remove a group of remote candidates from the ICE Agent. */
+- (void)removeIceCandidates:(NSArray<RTCIceCandidate *> *)candidates;
+
 /** Add a new media stream to be sent on this peer connection. */
 - (void)addStream:(RTCMediaStream *)stream;