We currently compute this value from the size of the external table,
which we get from the Isolate. This size is isolate-independent though,
so it can just be a constant.

R=mstarzinger@chromium.org

Bug: chromium:912043
Change-Id: If1c09a56b1a985b855f5b65818322979c194d772
Reviewed-on: https://chromium-review.googlesource.com/c/1362954Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58045}