This adds support for custom weak callbacks through static callbacks and
instance methods.

Bug: chromium:1056170
Change-Id: Ie4bd32539e0d933b192f07edb2d45e0070c2128d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2148784
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67167}