Verification code in HeapObject::set_map() is supposed to run on the
main thread since object layout change is only supported on the main
thread. There are some users of set_map() on background threads though,
which resulted in crashes. Since those users all perform a safe map
transition, we introduce a separate method for this purpose:
HeapObject::set_map_safe_transition(). This method behaves just like
set_map() but verifies that this is a safe map transition and not an
object layout change and therefore can be used on background threads
as well.

This CL also adds a DCHECK to HeapObject::set_map() to ensure we run
this method only on the main thread.

Bug: chromium:1293484
Change-Id: I25de6fda08de21b8b7a3645cf0ea5b1334e8a2f6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3439905Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78978}