Each page has a `live_byte_count_` field in its header and it is always accessed
with atomic operations. However, apart from one corner case, this field is only
accessed by the main thread so let's remove the atomicity by default.

To summarise, this changes the MarkingState accessors in the following way:

- IncrementalMarkingState:

Accesses of live_byte_count_ are now non-atomic. Concurrent marking uses local
maps of live bytes per page so we have exclusive access to the field when using
this.

- MajorAtomicMarkingState:

This accessor does not guarantee exclusive access to live_byte_count_ so we need
to keep the access atomic. This is used by the scavenger when migrating an
object that was marked black, since marking an object black will implicitly
increment the live_byte_count_.

Additionally, the live_bytes() and SetLiveBytes() accessor methods were not used
so remove them.

- MajorNonAtomicMarkingState:

Accesses of live_byte_count_ are not actually non-atomic.

- MinorNonAtomicMarkingState:

The `young_generation_live_byte_count_` field is atomic so we can set the memory
order manual to relax accesses.

Change-Id: I4c6457843783157ca878540e34ad50878afee20c
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Reviewed-on: https://chromium-review.googlesource.com/c/1456095Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#59548}