The elements of a {WasmInstanceObject} were not visited during GC,
leading to crashes when using them later.
This CL fixes this by visiting the whole {JSObject} header, consisting
of properties and elements.

R=titzer@chromium.org

Bug: chromium:839919, chromium:946350
Change-Id: I070fb3e6a7fd87a7288fc68b284100a2f9c72e9a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1541237
Auto-Submit: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60488}