Tiny simplification of FieldIndex::ForInObjectOffset.
Tiny simplification of FieldIndex::ForInObjectOffset.

Review URL: https://codereview.chromium.org/884163004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#26462}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh