AllocationSites are strongly rooted in various places.
AllocationMementos, small heap objects immediately behind the
objects which AllocationSites tracks, are purposely left
unrooted. They do however, point to AllocationSites.

This leads to a situation where an AllocationSite may no longer
be seen to have strong roots at gc time, and yet new space is
still repleat with AllocationMementos which point to it.
The GC recognizes this, and marks the AllocationSite as a
"zombie," that is, an object which should be kept alive for
one more GC cycle because of the existence of those mementos
which point to it.

Change-Id: Ifa720c28f216dee2eaf7edd6f489b5c7427d4353
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2928500Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74866}