New space objects which die after scavenging might contain weak references.
IncrementalMarking::UpdateWeakReferencesAfterScavenge must drop the
corresponding slot.

This bug didn't surface before, since all weak slots are in the old space (but
this will change soon).

BUG=v8:7308

Change-Id: Ib1e507d4207e35547240dc0867ec7787b3f3103e
Reviewed-on: https://chromium-review.googlesource.com/1005000Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52519}