Introduce empty rtc_base:async_resolver target.
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 <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Reviewed-by: Harald Alvestrand <ce816cd73d80f958d82de1df356058c2d2bca378@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#32829}

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