This is a reland of 0c2faa06
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}

Bug: chromium:756050,v8:7146
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I8fe3a9d9af43196e16d02342a47347a76c0a1341
Reviewed-on: https://chromium-review.googlesource.com/811724Reviewed-by: Eric Holk <eholk@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49945}