The instance_size of a live map can change via ShrinkInstanceSize. This
change was outside of the scope of the MapUpdater. In order to have a
consistent view of the data, the concurrent reader will access the map
updater lock if needed.

Also refactor MapUpdaterMutexDepthScope (now named
`MapUpdaterGuardIfNeeded`) so that A) it's not possible to forget to
lock it, and B) add V8_NODISCARD to the class.

As a second refactor use std::function in TraverseCallback.

Bug: v8:7790
Change-Id: I57dd00699ccb1c9f132a950db93704b07ca115ac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2862765Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74592}