Include rtc_base/async_resolver.h (WebRTC types migration).

In https://webrtc-review.googlesource.com/c/src/+/196903, WebRTC is
refactoring //rtc_base:rtc_base and rtc::AsyncResolver will be moved
to a new header. The new header has been introduced by
https://webrtc-review.googlesource.com/c/src/+/197941.

Bug: webrtc:9987
Change-Id: I7de84e369ba5cded63b536f3b086b797a434db7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593640Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837966}

remoting/protocol/chromium_socket_factory.cc

@@ -28,6 +28,7 @@
 #include "remoting/protocol/stream_packet_socket.h"
 #include "third_party/webrtc/media/base/rtp_utils.h"
 #include "third_party/webrtc/rtc_base/async_packet_socket.h"
+#include "third_party/webrtc/rtc_base/async_resolver.h"
 #include "third_party/webrtc/rtc_base/net_helpers.h"
 #include "third_party/webrtc/rtc_base/socket.h"