This change makes it possible to enable SharedArrayBuffer per Context,
controlling whether it should be enabled or not with a callback. The
previous implementation of the reverse origin trial for
SharedArrayBuffer was broken, since the feature could only be enabled
globally per process, and only if the feature flag is set early enough
in the v8 initialization. This does not play well with how origin
trials work.

The implementation is similar to the callbacks that already exist for
the origin trials for WebAssembly simd and exceptions.

SharedArrayBuffer is still controlled by the flag
harmony_sharedarraybuffer. If that flag is disabled, then
SharedArrayBuffer is disabled unconditionally. On top of that, this CL
introduces a new flag for enabling SharedArrayBuffer per context. If
that flag is set, a callback is used to determine whether
SharedArrayBuffer should be enabled.


Note that this only controls whether the SharedArrayBuffer constructor
should be exposed on the global object or not. It is always possible
to construct a SharedArrayBuffer using

  new WebAssembly.Memory({
    shared:true, initial:0, maximum:0 }).buffer.constructor;


There are few things which I do not like of this approach, but I did
not have better ideas:

1. The complex logic of dobule flag + callback. However, this seemed
the best way to me to not break embedders which rely on that flag
being enabled by default.

2. The fact that what actually matters is just whether the callback
returns `true` once. It would be good to check that the callback gives
a consistent return value, or to provide a better API that cannot be
missunderstood.


Bug: chromium:923807,chromium:1071424,chromium:1138860
Change-Id: Ibe3776fad4d3bff5dda9066967e4b20328014266
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2867473Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Antonio Sartori <antoniosartori@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74378}