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/examples/objc/AppRTCDemo/ARDSignalingMessage.m

@@ -16,7 +16,8 @@
 #import "RTCIceCandidate+JSON.h"
 #import "RTCSessionDescription+JSON.h"
 
-static NSString const *kARDSignalingMessageTypeKey = @"type";
+static NSString * const kARDSignalingMessageTypeKey = @"type";
+static NSString * const kARDTypeValueRemoveCandidates = @"remove-candidates";
 
 @implementation ARDSignalingMessage
 
@@ -47,6 +48,12 @@ static NSString const *kARDSignalingMessageTypeKey = @"type";
     RTCIceCandidate *candidate =
         [RTCIceCandidate candidateFromJSONDictionary:values];
     message = [[ARDICECandidateMessage alloc] initWithCandidate:candidate];
+  } else if ([typeString isEqualToString:kARDTypeValueRemoveCandidates]) {
+    RTCLogInfo(@"Received remove-candidates message");
+    NSArray<RTCIceCandidate *> *candidates =
+        [RTCIceCandidate candidatesFromJSONDictionary:values];
+    message = [[ARDICECandidateRemovalMessage alloc]
+                  initWithRemovedCandidates:candidates];
   } else if ([typeString isEqualToString:@"offer"] ||
              [typeString isEqualToString:@"answer"]) {
     RTCSessionDescription *description =
@@ -84,6 +91,27 @@ static NSString const *kARDSignalingMessageTypeKey = @"type";
 
 @end
 
+@implementation ARDICECandidateRemovalMessage
+
+@synthesize candidates = _candidates;
+
+- (instancetype)initWithRemovedCandidates:(
+    NSArray<RTCIceCandidate *> *)candidates {
+  NSParameterAssert(candidates.count);
+  if (self = [super initWithType:kARDSignalingMessageTypeCandidateRemoval]) {
+    _candidates = candidates;
+  }
+  return self;
+}
+
+- (NSData *)JSONData {
+  return
+      [RTCIceCandidate JSONDataForIceCandidates:_candidates
+                                       withType:kARDTypeValueRemoveCandidates];
+}
+
+@end
+
 @implementation ARDSessionDescriptionMessage
 
 @synthesize sessionDescription = _sessionDescription;