[SxS] Add split view ID property for extensions API

Missed this reference when creating the initial CL.

Change-Id: Ie7bf1150793aa26797225c60200651fb1053101c
Bug: 406794014
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6780030
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Alison Gale <agale@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1491700}
NOKEYCHECK=True
GitOrigin-RevId: 5a68de57ae41ec6aba04f5db77c38be20fc47706