This CL implements the above in a concurrent setting without relying
on serialization (except existing serialization to read a consistent
JSObject state, which should be addressed in future work).

There are three main cases in which GetOwnConstantElement can succeed:

- Frozen elements are always constant. The backing store is immutable
after initialization and can be accessed through relaxed reads.
- String wrapper elements are always constant. The JSPrimitiveWrapper
is immutable after initialization, and internalized Strings are
protected by a mutex (other string kinds are currently not handled).
- Dictionary elements may be constant. Since this case is not
particularly important for the optimization, we leave it unimplemented
for now.

Bug: v8:7790
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_no_cm_rel_ng
Change-Id: If2fbced50218ebd3930da8157cd2ae5eb83a8e02
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2717308Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73442}