We convert this {ptr, size} pair to an ArrayBuffer::Allocation when we
need to free it anyway, so we can get rid of this intermediate step to
make things simpler.

Change-Id: I6e82949ec02acb5794f4d668afb2313ebdcb9d52
Reviewed-on: https://chromium-review.googlesource.com/1136309
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54457}