Currently dead ArrayBuffers with external memory are discovered and
freed in sweeper. If sweeping is slow, it can take long time until
the external memory is freed. This is why wasm currently forces
at least two back-to-back GCs on allocaiton failure.

This patch ensures eager finalization of ArrayBuffers on critical
memory pressure notification.

Bug: v8:7621
Change-Id: I96c75922577388cafab049f5cbce01d636a7b400
Reviewed-on: https://chromium-review.googlesource.com/1224328
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55908}