Use the existing macros to load MemoryStart and MemorySize from the
instance.

R=titzer@chromium.org

Change-Id: Iaa597fedcfc4581503d7cdf2fb94da6e285cc545
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1538122Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60434}