- 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}