Concurrently process objects and only read embedder fields on the main
thread.

Also prepares the concurrent marking infrastructure to plug this
processing into different types.

Bug: chromium:885125, chromium:843903
Change-Id: I23b7f778c16cff118dec93e11e2bbd02aaf11a78
Reviewed-on: https://chromium-review.googlesource.com/1231175Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56043}