Add AsyncResolverFactory interface and basic implementation.

The factory is plumbed down to P2PTransportChannel and will eventually
be used to resolve hostnames. Uses of PacketSocketFacotry::CreateAsyncResolver
will eventually be migrated to use this factory instead.

Bug: webrtc:4165
Change-Id: I1c48b2ffb8649609a831eba291f67ce544bb10eb
Reviewed-on: https://webrtc-review.googlesource.com/91300
Commit-Queue: Zach Stein <zstein@webrtc.org>
Reviewed-by: Emad Omara <emadomara@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Qingsi Wang <qingsi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24176}

api/asyncresolverfactory.h

@@ -0,0 +1,33 @@
+/*
+ *  Copyright 2018 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.
+ */
+
+#ifndef API_ASYNCRESOLVERFACTORY_H_
+#define API_ASYNCRESOLVERFACTORY_H_
+
+#include "rtc_base/asyncresolverinterface.h"
+
+namespace webrtc {
+
+// An abstract factory for creating AsyncResolverInterfaces. This allows
+// client applications to provide WebRTC with their own mechanism for
+// performing DNS resolution.
+class AsyncResolverFactory {
+ public:
+  AsyncResolverFactory() = default;
+  virtual ~AsyncResolverFactory() = default;
+
+  // The returned object is responsible for deleting itself after address
+  // resolution has completed.
+  virtual rtc::AsyncResolverInterface* Create() = 0;
+};
+
+}  // namespace webrtc
+
+#endif  // API_ASYNCRESOLVERFACTORY_H_