Introduce a way to set a custom finalization callback that can be used
to signal and set up destruction of embedder memory.

Bug: chromium:923361
Change-Id: Ifc62ebd534aba3b02511c74b59161ec3edc0ee0d
Reviewed-on: https://chromium-review.googlesource.com/c/1452447
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59381}