Fix FATAL crash when creating a new tab group

Set the MaximumFlexSizeRule in the FlexSpecification of the
ColorPickerView layout manager to kPreferred. This prevents
ColorPickerView from expanding beyond the preferred size of its child
views (ColorPickerElementView), ensuring they remain square and avoiding
layout inconsistencies.

Bug: 389559081
Change-Id: I1f0e0d099c3842161624abc7142356d19092701f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6278645
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1426402}