Before this patch, both key and value of an ephemeron pair was always
considered to be GarbageCollected objects.

This patch adjusts the snapshotting mechanism to accomodate that
values may not be GarbageCollected objects and must thus be eagerly
traced for visibility and edge creation.

In practice this only shows up in Blink when associating an existing
wrappable with a wrapper in a non-main world, e.g., through an
extension. In this case, DOMWrapperMap keeps the wrapper value through
a TracedReference in the ephemeron map with the existing wrappable as
key. The semantics are intended to be general ephemeron semantics,
i.e., value needs to be kept alive when the key is alive.  This is
visualized in DevTools as the main wrapper/wrappable pair (which is
merged into a single node for the snapshot) retaining the non-main
world wrapper.

Bug: chromium:1245894
Change-Id: Ibfa6722f20c76f94c310f9a040f0d3d4b9083bbb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3140601Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76658}