It sometimes meant just the field and sometimes the computed byte
length. This rename makes it less confusing.

Bug: v8:11111
Change-Id: I64bb8b6b9be2befb57d53ae5456b6ce8f6472456
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3657429Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80695}