This increases the maximum nesting level for memory modification scopes
from 3 to 4. It is a follow-up to WebAssembly optimizations which did
increase the total nesting in favor of performance. This also hoists
out the value into a constant, so that it is easier to change.

R=ahaas@chromium.org
BUG=v8:6792,chromium:787731

Change-Id: Ib60a7d66cdf42227d6b717a38c0923bcbbacf8dc
Reviewed-on: https://chromium-review.googlesource.com/788859Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49626}