Most NativeContext elements are immutable after initialization;
additionally, we now use acquire-release semantics to load/store
elements when possible. Reading and constructing Refs for elements
is thus possible from the background.

A few notes:

- A few elements are not immutable; if read from the background
thread, these must use acquire-release semantics.
- Elements can be stored from generated code; these are not compatible
with bg-thread accesses.
- While elements can be read safely from the native context, the
elements themselves may still require serialization; this is done in
NativeContextRef::Serialize.

Bug: v8:7790
Change-Id: I12e9611a292e7dd912438c712390731a5422407d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2897254
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74604}