Enable DHT client in WebTorrent for supporting trackless torrents

[yrliou]

In bittorrent-dht, it's using nodejs's dns.lookup, we should provide our own shim to use chromium's existing API or implemented one if none exists.

[yrliou]

QA note: See brave/brave-core#443 for the test plan.

[srirambv]

Verification Passed on

Brave	0.55.11 Chromium: 70.0.3538.35 (Official Build) beta (64-bit)
Revision	28dcb499844fa40c28d5f62e337876cb936f79f5-refs/branch-heads/3538@{#678}
OS	Linux and Windows

• Verified webtorrent is loaded and able to connect to peers and download the file

Verified passed with

Brave	0.55.11 Chromium: 70.0.3538.35 (Official Build) beta(64-bit)
Revision	28dcb499844fa40c28d5f62e337876cb936f79f5-refs/branch-heads/3538@{#678}
OS	Mac OS X

• modified test plan from Enable DHT client in WebTorrent brave-core#443 slightly, due to Pasting magnet link in URL does search instead of loading web torrent #1435. Verified issue by opening Dev Tools and running the following command: window.open("magnet:?xt=urn:btih:dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&dn=Big+Buck+Bunny"). Then I was able to start the download and locate peers.