- Introduces a API to set top of the stack through
  EmbedderHeapTracer::SetStackTop.
- Introduces a new API to inform V8 about an empty embedder stack.
- Switch internal representation of TracedReference
  for on-stack handles to a proper stack that considers all
  contained handles as roots.
- Handle garbage is avoided by cleaning up on handle creation or
  GC.

Design doc: https://bit.ly/on-stack-traced-reference

Bug: chromium:1040038
Change-Id: I927ef0abb268fdb5853c9e17b1bc96e2491cf101
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1993973
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65757}