Add testing of switching appearance on customizable- picker

This new test exercises the appearance property on the picker of with appearance:base-select. In particular, it tries to change the picker appearance while the picker is open. The desired behavior is that doing this closes the picker, and does not re-open it, to avoid both hysteresis and oscillation.

This currently hits a few CHECKs in different areas of the codebase, so I can't really evaluate whether all of the test is accurate. Once the crashers are fixed, we can see.

Bug: 364924715, 364924718 Change-Id: Ief448bdb0cf9d3d37032ff7bbc257fcea00a1ccc Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5840689 Reviewed-by: David Baron dbaron@chromium.org Commit-Queue: Mason Freed masonf@chromium.org Auto-Submit: Mason Freed masonf@chromium.org Cr-Commit-Position: refs/heads/main@{#1352262}