For many subclasses of JSObject, we used kSize instead of kHeaderSize
even though they can contain in-object properties. In fact, kSize
was very much used as the header size, as can be seen in many examples
in this CL.

This change is a preparation for a for a cleanup of how Torque
generates field offsets.

TBR=hpayer@chromium.org

Change-Id: I350e996057cd66c427381334080f8ac93de88597
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1917141
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65013}