Avoid clearing the memory on the embedder-side of a TracedGlobal handle.

When using destructors in TracedGlobal this is safe as long as the embedder
reports the handle on tracing GCs. If the embedder does not report a handle it
is assumed that the containing object is dead as well.

Without using destructors the same argument holds for tracing GCs. In addition,
embedders using the optimization of clearing references on non-tracing GCs
are expected to clear the reference in ResetHandleInNonTracingGC.

It is suggested that only expert embedders make use of (a) no destructors and
(b) IsRootForNonTracingGC.

Change-Id: Ia417c0eb0860094fcaa554e7046d38abac905714
Bug: chromium:995684
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1763539
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63362}