This reverts commit 9aba0159.

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

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

in MarkCompactCollector::ProcessEphemeronMarking(). See e.g. https://logs.chromium.org/v/?s=chromium%2Fbb%2Fclient.v8.fyi%2FV8-Blink_Linux_64__dbg_%2F12321%2F%2B%2Frecipes%2Fsteps%2Fwebkit_unit_tests%2F0%2Fstdout and more on https://ci.chromium.org/p/v8/g/fyi/console

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}

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

Change-Id: Ib44bfe8c49e8fc30c3d0f2beba03a2895530dfd6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:844008
Reviewed-on: https://chromium-review.googlesource.com/1116118Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54045}