Allow mocking the limits for ArrayBuffer allocation to simulate operating
system OOM.

Fixes:
- Ensure OS limit > hard limit for external memory. This is necessary as
  any processing below the hard limit is opportunistic. E.g. a running
  sweeper may stall the current marking (GC) round.
- Immediately process AB allocations when under memory pressure. Otherwise,
  the allocations may be stuck in a stalled task. Freeing them upon
  adding them to the collector still enables parallelism if possible.

This reverts commit f3ad6cdb.

Bug: chromium:845409
Change-Id: Ic3e458f2af231bae3d53afcfd6002a0347d3f12b
Reviewed-on: https://chromium-review.googlesource.com/1206872
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55656}