Introduce empty rtc_base:async_resolver target.

This is needed in order to prepare Chromium to depend on the new
target to land (https://webrtc-review.googlesource.com/c/src/+/196903).

No-Try: True
Bug: webrtc:9987
Change-Id: Ic7337f1c57c135d541eb424fb06dae32acacaea8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/197801
Commit-Queue: Mirko Bonadei <[email protected]>
Reviewed-by: Harald Alvestrand <[email protected]>
Cr-Commit-Position: refs/heads/master@{#32829}

Author: MirkoBonadei

rtc_base/BUILD.gn

@@ -665,6 +665,12 @@ rtc_library("rtc_json") {
   }
 }
 
+rtc_source_set("async_resolver") {
+  # TODO(bugs.webrtc.org/9987): This build target will soon contain
+  # async_resolver source files (see
+  # https://webrtc-review.googlesource.com/c/src/+/196903).
+}
+
 rtc_source_set("net_helpers") {
   # TODO(bugs.webrtc.org/9987): This build target will soon contain
   #                             the following files: