snap-group: Limit the divider position within the allowed range

Currently, the divider position can go beyond the window minimum
size. This cl did the pre-calculation for the divider allowed
position range before updating the divider bounds.

Demo: http://b/304367964#comment10
Bug: b/304367964
Change-Id: I5b77f0bb01d1266bfe4151833f2c85a3ae6c09ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5378785


Commit-Queue: Michele Fan <michelefan@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1276996}