When marking ephemerons visit individual ephemerons with key and value
unreachable instead of simply iterating all ephemerons in all weak
collections. Also visit ephemerons at end of concurrent marking to do
work we would otherwise need to do in the atomic pause.

Bug: chromium:844008
Change-Id: I3400ad1f81c0cdc0fe6506a1f1146a6743a7fcd7
Reviewed-on: https://chromium-review.googlesource.com/1113934
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54039}