snap-group: Finalize snap ratio diff value

Fixed: b/331238902
Change-Id: Ibd259b35d69839d8c6ab7e57cfb0781dc55e3efb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5406329


Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Michele Fan <michelefan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280236}