Embedders forward the Value in TraceEphemeron as Member reference (as
depicted in the API docs). Add TraceTrait<Member<T>> that forwards to
TraceTrait<T> accordingly, supporting the intended use case.

Bug: chromium:1056170
Change-Id: I3b247cb3553ae34d9ff5393aefeaec24068e78c2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2656255
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72412}