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}

rtc_base/bytebuffer.h

@@ -185,7 +185,7 @@ class ByteBufferReader : public ByteBuffer {
   // after this call.
   bool Consume(size_t size);
 
- private:
+ protected:
   void Construct(const char* bytes, size_t size);
 
   const char* bytes_;
@@ -193,6 +193,7 @@ class ByteBufferReader : public ByteBuffer {
   size_t start_;
   size_t end_;
 
+ private:
   RTC_DISALLOW_COPY_AND_ASSIGN(ByteBufferReader);
 };