[Compare] Prevent comparison table list from shrinking

This patch prevents the comparison table list from shrinking due to
flexbox constraints. The container would previously shrink if all items
were removed and only the toast was visible, but setting `flex-shrink`
to 0 ensures the width is not changed by flexbox sizing.

Before:
http://go/scrcast/NTA5NDk0MDE4MTc5MDcyMHw1N2YwMzRmZC03ZA
After:
http://go/scrcast/NDYxMjUzNzUwODE2NzY4MHw3MjM1MjM5Zi0wOA

Bug: 393155297
Change-Id: Ida100f3d36ab68f5209d5049f3a215b41ea52d6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6214711
Commit-Queue: Chase Hartsell <chazzy@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1413143}