This reverts commit 91f113e2.

Reason for revert: still causes breakage on Chromium-integrated builds, failing here:

  CHECK_EQ(0, heap()->local_embedder_heap_tracer()->NumberOfWrappersToTrace());

Original change's description:
> Reland "[heap] Visit individual ephemerons instead of collections"
>
> This is a reland of 9aba0159
>
> Original change's description:
> > [heap] Visit individual ephemerons instead of collections
> >
> > 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}
>
> Bug: chromium:844008
> Change-Id: I4c44e74c7cf5fe380ffa4ce9f106bebb57bc023d
> Reviewed-on: https://chromium-review.googlesource.com/1116438
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Dominik Inführ <dinfuehr@google.com>
> Cr-Commit-Position: refs/heads/master@{#54046}

TBR=ulan@chromium.org,dinfuehr@google.com

Change-Id: I4a059e86ba06e0b2562afc311b12d397cd78e857
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:844008
Reviewed-on: https://chromium-review.googlesource.com/1116718Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54047}