This change enables clean-up of weak map entries in the
scavenger of the weak map is in the young generation.
With this change, the scavenger treats keys in ephemerons as
weak instead of strong, but does not implement full ephemeron
semantics: Values are treated always as strong, independently
of whether the key is live or not.

This approach ensures that no value is cleaned up accidentally.
After scavenging, all entries with dead keys are removed from
weak maps. After that, some values that are not referenced anywhere
anymore might still be in the heap, and those can be cleaned up
in the next scavenge.

What the scavenger does, amounts to one iteration of the
fixed-point algorithm required to implement ephemeron semantics.
We hope that this is a reasonable trade-off between time spent
tracing and cleaned-up entries.

This change does not affect weak maps that reside in old space.

Bug: v8:8557
Change-Id: Ic5618b3b863ad8c314c87449571150e756fecbf0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1467182
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60101}