This reverts commit b6658ade.

Reason for revert: Breaks TSAN :(

Original change's description:
> [heap] Concurrently free ArrayBuffer allocations.
> 
> Free ArrayBuffer backing stores on a background thread, rather than
> blocking the main thread after processing. Could potentially cause
> contention with the array buffer allocator once JS execution resumes.
> 
> The new ArrayBufferCollector class tracks these dead allocations.
> 
> Later, the processing of array buffers can happen in parallel.
> 
> Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
> 
> Bug: v8:6992
> Change-Id: I49ae4db12ed62d8400ba2bbafeda05a11479d904
> Reviewed-on: https://chromium-review.googlesource.com/739829
> Commit-Queue: Peter Marshall <petermarshall@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49485}

TBR=hpayer@chromium.org,mlippautz@chromium.org,petermarshall@chromium.org

Change-Id: If6743b83f871c0fd0d6e83a3083dce0eecd99021
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6992
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/779159Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49488}