The debugger maintains a stack of promises used for catch prediction
with promise builtins and async functions. Previously this stack would
hold on to the individual promises strongly, and subtle bugs that lead
to not properly cleaning up the stack in some corner cases would often
lead to significant memory issues (e.g. leaking whole iframes).

This refactors the PromiseOnStack to be

  (a) on the V8 heap, rather than allocating C++ structs with global
      handles pointing to the promises, and
  (b) hold on to the promises only weakly.

While this will not guarantee proper promise stack management, it will
at least ensure that edge cases don't lead to catastrophic (debugger
only) leaks.

Bug: chromium:1292063
Change-Id: I9c293ca2032de3a59e1e9624f132d37187805567
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3545176
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79594}