ComputeMinObjectSlack is called concurrently from background threads
(when --concurrent-inlining) and must therefore be thread-safe.

This CL adds a compiler-specific thread-safe variant
of ComputeMinObjectSlack in addition to the plain old non-thread-safe
one. Thread-safety is achieved through locking: on the bg thread, a
shared lock when traversing transitions, and on the main thread, an
additional exclusive critical section when overwriting prototype
transitions.

Tbr: leszeks@chromium.org
Bug: v8:7790,v8:12010,chromium:1231901
Change-Id: If5af83df1ab896b22477921449fb5ba4c8d3e8a3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3045342
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75949}