ArrayBuffer memory allocated off-heap was previously tracked by a test-
only retained_size() field on each LocalArrayBufferTracker.

Changes in off-heap ArrayBuffer memory usage are now reported to the
Space with which the ArrayBuffer is associated, so that the value is
cheaply available to include in e.g. GC limit calculations, via a new
getter, ExternalBackingStoreBytes().

Changes to external ArrayBuffer backing-store allocations are tracked in
an AtomicNumber associated with each Space, to allow for ArrayBuffers
being concurrently moved or freed from multiple Pages in the same Space
during sweeps & compactions.

Bug: chromium:837583
Change-Id: I8b1b6addd5cd05533d8da55ca813e134bc36e181
Reviewed-on: https://chromium-review.googlesource.com/1052347
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53192}