For more static type safety: Avoid passing wasm objects as Object and
casting them before use. Use the correct type right away.

R=ahaas@chromium.org
BUG=v8:6474

Change-Id: Id0c486560115dd1a7bd9b6a12d2fb938e06520ef
Reviewed-on: https://chromium-review.googlesource.com/530744Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45838}