This has benefits both in terms of performance and memory usage. Aside from
the obvious savings of not loading additional JS scripts in every process,
this also allows us to move more of our expensive data collection work to a
background thread, where it doesn't risk janking both parent and content
processes.
MozReview-Commit-ID: 2A593R7bIKB
Differential Revision: https://phabricator.services.mozilla.com/D13872
509e210f19