This reverts commit 5611f70b.

Reason for revert: flaky tests: v8:9588, v8:9587

Original change's description:
> "Reland x4 [arraybuffer] Rearchitect backing store ownership"
> 
> This is a reland of bc33f5ae
> 
> Contributed by titzer@chromium.org
> 
> Original change's description:
> > [arraybuffer] Rearchitect backing store ownership
> >
> > This CL completely rearchitects the ownership of array buffer backing stores,
> > consolidating ownership into a {BackingStore} C++ object that is tracked
> > throughout V8 using unique_ptr and shared_ptr where appropriate.
> >
> > Overall, lifetime management is simpler and more explicit. The numerous
> > ways that array buffers were initialized have been streamlined to one
> > Attach() method on JSArrayBuffer. The array buffer tracker in the
> > GC implementation now manages std::shared_ptr<BackingStore> pointers,
> > and the construction and destruction of the BackingStore object itself
> > handles the underlying page or embedder-allocated memory.
> >
> > The embedder API remains unchanged for now. We use the
> > v8::ArrayBuffer::Contents struct to hide an additional shared_ptr to
> > keep the backing store alive properly, even in the case of aliases
> > from live heap objects. Thus the embedder has a lower chance of making
> > a mistake. Long-term, we should move the embedder to a model where they
> > manage backing stores using shared_ptr to an opaque backing store object.
> 
> TBR=yangguo@chromium.org
> 
> BUG=v8:9380,v8:9221,chromium:986318
> 
> Change-Id: If671a4a9ca0476e8f084efae46e0d2bf99ed99ef
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1731005
> Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#63041}

TBR=ulan@chromium.org,yangguo@chromium.org,mstarzinger@chromium.org,clemensh@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:9380, v8:9221, chromium:986318
Change-Id: Ic7381239f4e90d0c437b7e47a5ac6e8bce60f882
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1736747Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63081}