message: 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
  Cr-Commit-Position: refs/heads/master@{#32829}
diff: @@ -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:
  