This reverts commit 0c2faa06.

Reason for revert: seems to make win asan flaky:
https://build.chromium.org/p/client.v8/builders/V8%20Win32%20ASAN/builds/1094
https://build.chromium.org/p/client.v8/builders/V8%20Win32%20ASAN/builds/1095

Original change's description:
> [D8] Clean up ArrayBuffer Allocators in shell.
> 
> - Reworks the class hierarchy in d8.cc to conform to the allocator API.
>   In particular, allocators should malloc/free or equivalent unless
>   v8::ArrayBuffer::Allocator::Reserve is called.
> - Modifies ExternalizedContents to remember the allocation mode.
> - ArrayAllocatorBase now tracks its allocations to make sure it doesn't
>   call Free on externalized array buffers it didn't allocate.
> 
> Bug: chromium:756050
> Change-Id: Ic2d07d36358f1b4fa542bea27f93d1d51a1757e1
> Reviewed-on: https://chromium-review.googlesource.com/807355
> Commit-Queue: Bill Budge <bbudge@chromium.org>
> Reviewed-by: Ben Titzer <titzer@chromium.org>
> Reviewed-by: Eric Holk <eholk@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49893}

TBR=bbudge@chromium.org,titzer@chromium.org,eholk@chromium.org

Change-Id: I0f3ecc10de843102e7681285f0300cf98704c92c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:756050
Reviewed-on: https://chromium-review.googlesource.com/810904Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49895}