Implement MULTICAST_DNS source for new HostResolver API.

Only used when explicitly requested. The MDnsClient is created and
starts listening for MDNS messages at first use in HostResolverImpl.

For now supports only a basic feature set, eg we'll DCHECK if canonname
is requested (in contrast to the async resolver behavior that was
unnoticed until recently due to the lack of a DCHECK).

Bug: 846423
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Ib296ac432d392baee2454587693b2537513f461c
Reviewed-on: https://chromium-review.googlesource.com/1211542
Reviewed-by: Matt Menke <[email protected]>
Reviewed-by: Tom Sepez <[email protected]>
Commit-Queue: Eric Orth <[email protected]>
Cr-Commit-Position: refs/heads/master@{#595956}
diff --git a/net/dns/host_resolver_source.h b/net/dns/host_resolver_source.h
index 96af9b2..d873c07 100644
--- a/net/dns/host_resolver_source.h
+++ b/net/dns/host_resolver_source.h
@@ -21,7 +21,8 @@
   // Results will only come from DNS queries.
   DNS,
 
-  // TODO(crbug.com/846423): Add MDNS support.
+  // Results will only come from Multicast DNS queries.
+  MULTICAST_DNS,
 };
 
 }  // namespace net