Implement network monitor for iOS.

Notably, this should detect whether an interface is "available" or not,
which should prevent the failure is with dual SIM card setups.

This is gated behind a field trial for now, to ensure this doesn't cause
any regressions due to false negatives (interfaces that are usable
but not listed as available by NWPathMonitor).

Bug: webrtc:10966
Change-Id: Ia3942c4c57b525d08d8b340e2325f3705cfd0304
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/180923
Commit-Queue: Taylor <deadbeef@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Jonas Oreland <jonaso@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31977}

sdk/objc/components/network/RTCNetworkMonitor.h

@@ -0,0 +1,24 @@
+/*
+ *  Copyright 2020 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#import <Foundation/Foundation.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+/** Listens for NWPathMonitor updates and forwards the results to a C++
+ *  observer.
+ */
+@interface RTCNetworkMonitor : NSObject
+
+- (instancetype)init NS_UNAVAILABLE;
+
+@end
+
+NS_ASSUME_NONNULL_END