- Add a new broker mode kRetired, in which the heap can
  again be accessed.
- Change the way modes work. We now always start in kDisabled.
  If FLAG_concurrent_compiler_frontend is on, we eventually move
  to kSerializing, then to kSerialized, then to kRetired.
- Add an ObjectDataKind to ObjectData that indicates whether the
  data is just a dummy (i.e. created while broker was in kDisabled
  mode).

This also happens to fix a bug found by clusterfuzz.

Bug: v8:7790, chromium:889722
Change-Id: I38833fe7ad26d2d3efb15ba560576defb82f673a
Reviewed-on: https://chromium-review.googlesource.com/1245425
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56260}