When the memory pressure level is critical and there are managed objects
that call AdjustAmountOfExternalMemory in their finalizer, we trigger
GC for each dying managed object. See the test for an example.

This fixes the bug by clearing the memory pressure level before GC.

Bug: v8:8014
Change-Id: Id5144430a52fb8545aa23f33229a11b1714cbf10
Reviewed-on: https://chromium-review.googlesource.com/1169011Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55021}