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/1116438Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Cr-Commit-Position: refs/heads/master@{#54046}