This is to ensure the snapshot is deterministic.

Internal fields can contain:
- reference to heap object
- embedder-defined aligned pointer
- a smi

The latter two are not distinguishable by V8, so if the serializer
callback returns non-zero value, we consider it to be an aligned pointer
and clear it to ensure that the snapshot does not contain memory
addresses that may not be deterministic. If the callback returns
{ nullptr, 0 } as result, we consider it to be a smi or some in-place
data that we then serialize verbatim.


R=jgruber@chromium.org

Bug: chromium:870584
Change-Id: I3cf9abf135ffd28d8138fa32636b12596b076e13
Reviewed-on: https://chromium-review.googlesource.com/c/1304441
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57272}