Previously, for generating the snapshot, ephemerons containers were just
traced strongly, without handling their ephemeron pairs. This resulted
in the snbapshot missing out on all the value objects (as keys were
traced through regular Visit()).

The fix here
a) Adds ephemeron tracing;
b) Adds a flag to avoid showing the key being retained by the
   ephemeron collection;

Bug: chromium:1056170
Change-Id: I45cc95bf4876879fa78b83154b13f20751b262b9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2948889
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75044}