[mDNS] Add NetLog for mDNS / service_discovery_client_mdns.

Add option to pass in NetLog to socket used for mDNS.
Pass in ChromeNetLog from Chrome code service_discovery_client_mdns.

This will hopefully help us diagnose long standing Cast / mDNS
reliability issues.

The NetLog entries can be easily filtered by searching the IP endpoint
used for mDNS (defined in net/dns/mdns_client.cc?).

BUG=751207

Change-Id: Id649a03d498e077738c5e071214f31f6577401e6
Reviewed-on: https://chromium-review.googlesource.com/647490


Reviewed-by: Gene Gutnik <gene@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Commit-Queue: Derek Cheng <imcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501360}