Add test for Fluent scrollbars when no thumb color is set.

CL:5786520 added a new early return for
PaintedScrollbarLayerImpl::AppendThumbQuads when Fluent scrollbars are
enabled and no thumb color is set.

This CL adds a new unit test to check that the early return is hit when
these conditions are met.

Bug: 359623669
Change-Id: I8f68cb778e566217f10da42253e597d3988135b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5800411


Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Yaroslav Shalivskyy <yshalivskyy@microsoft.com>
Commit-Queue: Gaston Rodriguez <gastonr@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1344914}