Whether or not a store requires a write barrier depends on several
invariants within V8. Some flags can break these invariants. In
particular, it's not possible to use enable_single_generation with
incremental marking because marking barriers are omitted in places where
it is assumed an object will be allocated in the young generation.

This CL introduces a new flag, enable_unconditional_write_barriers,
which allows us to specify that full write barriers should always
happens. The main purpose of this is to support single generation GC
with incremental marking, but it can also aid as a debugging tool to
check for missed write barriers.

Bug: v8:10614
Change-Id: I3ab640436bcefc118c9c5c34765421cb9ea4896f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2270546Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Jake Hughes <jakehughes@google.com>
Cr-Commit-Position: refs/heads/master@{#68623}