…r=gw

Support hiding slices to better understand what's on which layer,
and to hide UI when not relevant.
Requires using a HTTP server due to cross-scripting.

Differential Revision: https://phabricator.services.mozilla.com/D67963

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/c034bb22ce039bd90289c6ca972b00206b069dfa