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/peerconnectioninterface.h

@@ -72,6 +72,7 @@
 #include <utility>
 #include <vector>
 
+#include "api/asyncresolverfactory.h"
 #include "api/audio/audio_mixer.h"
 #include "api/audio_codecs/audio_decoder_factory.h"
 #include "api/audio_codecs/audio_encoder_factory.h"
@@ -1129,6 +1130,7 @@ struct PeerConnectionDependencies final {
   PeerConnectionObserver* observer = nullptr;
   // Optional dependencies
   std::unique_ptr<cricket::PortAllocator> allocator;
+  std::unique_ptr<webrtc::AsyncResolverFactory> async_resolver_factory;
   std::unique_ptr<rtc::RTCCertificateGeneratorInterface> cert_generator;
   std::unique_ptr<rtc::SSLCertificateVerifier> tls_cert_verifier;
 };