Currently GC decrements the external memory counter directly bypassing
the AdjustAmountOfExternalMemory. This is inconsistent with array
buffer allocation, which actually uses the API to increment the counter.

Change-Id: I401087872213fdd60f1a40c99c8f459c14dc0608
Reviewed-on: https://chromium-review.googlesource.com/582008Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46835}