[iOS] Rename parameter

This CL renames the `to_index` parameter of the `MoveGroup` API to `before_index` as this API moves the WebStates of `group` to be before
the WebState at `before_index`

Change-Id: I5aff7d8bc13b0db8d5470924b8364083d24e0222
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5419293


Reviewed-by: Asami Doi <asamidoi@chromium.org>
Auto-Submit: Ewann Pellé <ewannpv@chromium.org>
Commit-Queue: Asami Doi <asamidoi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281763}