Add mDNS discovery to the Fletch compiler
Add mDNS discovery to the Fletch compiler

This adds the new command 'x-discover' which will discover devices with
the Fletch agent.

This also adds the option to run discovery when asked for the IP address
of a remote device.

R=wibling@google.com
BUG=

Review URL: https://codereview.chromium.org/1422853009 .

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