cppgc: Handle ephemerons with Mixin keys.

This was causing DevTools to crush whenever I took a heap snapshot.

Bug: chromium:1056170
Change-Id: Ice7b3039c21a3f902f242299939e92ba0e393c9f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2720307


Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73065}