When stylo is not enabled in Gecko, the global state initialization
still creates the style thread pool, even when it's not going to be
used. This wastes address space and a little memory.
See https://bugzilla.mozilla.org/show_bug.cgi?id=1374824#c38 and
following comment.
Now that we require rayon 0.7, we can register start and stop handlers
that take care of dealing with the Gecko profiler for rayon threads in
the style thread pool.
I wanted to add an environmental variable to disable the style sharing
cache for gecko, but the current pattern involves lazy_static!, which
involves an atomic operation on lookup, which is a bit hot to do each
time we try to share styles. This makes that work happen once per
process.
