Structs with zero fields weren't handled correctly, because the GC
has a requirement that each object occupies at least two pointers.

On the high end, Wasm structs accidentally had a limit of 255 pointers
including object header. This CL bumps that to the intended limit
of 999 fields (which is arbitrary and could be raised if needed).

Bug: v8:7748
Change-Id: I13a3f45b3ddb28023c76775da32be0d07ec2ffd0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2874653
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74436}