Add the multicast DNS message format.

This CL adds the utilities to generate and parse mDNS messages (RFC 1035
and RFC 6762).

TBR=phoglund@webrtc.org

Bug: webrtc:9605
Change-Id: Id6121c17926887cd3a41a2dfc829462fd15f3a4c
Reviewed-on: https://webrtc-review.googlesource.com/93241
Commit-Queue: Qingsi Wang <qingsi@google.com>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Qingsi Wang <qingsi@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Alex Loiko <aleloi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24505}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 7a03b3b..2ef79fa 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -413,6 +413,7 @@
     "ignore_wundef.h",
     "location.cc",
     "location.h",
+    "message_buffer_reader.h",
     "numerics/histogram_percentile_counter.cc",
     "numerics/histogram_percentile_counter.h",
     "numerics/mod_ops.h",