Bug 1588223 - Use many domains in iframes for tabswitch test r=stephendonner,perftest-reviewers,rwood

This adds a tp5n-multiproc output directory which contains postprocessed
versions of the tp5n set. They are processed to take iframes with relative
URLs and output absolute URLs with domains similar to their original
domains. This will help us better simulate a real-world case for Fission,
which will split processes based on the ETLD+1 of the domain.

Differential Revision: https://phabricator.services.mozilla.com/D53065

--HG--
extra : moz-landing-system : lando