GCs should only trigger only trigger when growing external memory but
not when removing it.

- The limit is already lowered when removing memory, so possible future
  allocations check against a lowered limit.
- Memory pressure signals are already handled via an explicit V8 API.

Bug: chromium:899035
Change-Id: I96da5862400e06edb8c9fa47357070b3b48560a1
Reviewed-on: https://chromium-review.googlesource.com/c/1301473Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57024}