.. by locking the MapUpdater lock during MapData construction.

Note this only applies to basic MapRef/MapData construction. Some
methods, in particular MapRef::SerializeFoo methods, are not yet
background-serializable in general and require more work.

Bug: v8:7790
Change-Id: I473e78c82012ab6abc5a0633a4d34c4a40a3fb77
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2839553
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74164}