This reverts commit df8e6177.

Reason for revert: Multiple flakes in apparently related areas:

https://logs.chromium.org/logs/v8/buildbucket/cr-buildbucket.appspot.com/8906409837768155568/+/steps/Check__flakes_/0/logs/BackingStoreTest.RacyGrowWasmMem.../0

Original change's description:
> "Reland x3 [arraybuffer] Rearchitect backing store ownership"
> 
> This is a reland of bc33f5ae
> 
> 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.
> 
> R=​mlippautz@chromium.org
> BUG=v8:9380,v8:9221,chromium:986318
> TBR=ulan@chromium.org
> 
> Change-Id: I6c49e2425029b5664ef1c68dab8b5146f4ed0ff2
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1719191
> Reviewed-by: Ben Titzer <titzer@chromium.org>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Commit-Queue: Ben Titzer <titzer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#63007}

TBR=mstarzinger@chromium.org,titzer@chromium.org,mlippautz@chromium.org

Change-Id: If0266e5893b1325a332d5986337fa7ece2cb6943
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:9380, v8:9221, chromium:986318
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1729549Reviewed-by: Francis McCabe <fgm@chromium.org>
Commit-Queue: Francis McCabe <fgm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63011}