Shared values need to be kept alive while being conveyed across threads (i.e. by
postMessage). Currently they are meant to be conveyed through the
serializer/deserializer by the embedder via API. This both clunky and
the embedder has no good choice for what to do, because the most natural
choice is v8::Global and it is not designed to be threadsafe.

This CL removes the API and transparently handles conveying shared
values by using a wrapper around PersistentHandles called
SharedValueConveyors. Any isolate can own the conveyor provided it
outlives the receipt of the message by the receiving isolate. For
simpler lifetime management, the shared isolate currently owns all
conveyors.

Bug: v8:12547
Change-Id: I8f71b2faa0f8a1973f8b97ffccf4f5ad230f4e16
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3857453Reviewed-by: Jakob Linke <jgruber@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82795}