Cppgc exposes EphemeronPair that contains a WeakMember key and a Member
value and can be used to denote ephemeron semantics in the standalone
library.
Tracing EphemeronPairs goes through TraceEphemeron that is exposed on
the api for the blink usecase.

Bug: chromium:1056170
Change-Id: I9fbaa284fa2034248cdf36ea8b0cd5be6a55f676
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2467842
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70525}