Evacuating and compacting LayoutDescriptor is meta-circular, i.e., we
need the descriptor to iterate the objects that are copied. Separate the
phases to avoid requiring a publishing store for object payloads.

Bug: v8:6884
Change-Id: Icc3bb80a938fbd9eec60c79d88ab8fdd9bda0840
Reviewed-on: https://chromium-review.googlesource.com/700437Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48288}