Moves the serialization of the initial set of maps read from the native
context from the forground serialization phase to a new background
serialization phase.

BUG=v8:7790,v8:9684

Change-Id: Id408e5c3096e832cf30106eb8fb7d65518432e06
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2613028
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72166}