Currently we have to access the unreachable ArrayBuffer object to get
the backing store pointer when we want to free it. This means we need
the original ArrayBuffer object to stay alive until we collect all of
the pointers to free (currently done sequentially after marking).

We want to move this step to a background task that does not block GC
finishing - to do that we need the backing store pointers so that the
original page (where the ArrayBuffers live) can be freed.

Change-Id: Ifaf070d777939cb23c46da637a25d75f9c863bd8
Reviewed-on: https://chromium-review.googlesource.com/1102434
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53915}