Add a "top" scroller position for customizable-<select>

This updates the way we do scrollable fallback positions, using the
`max-block-size: stretch` property. That eliminates the need for
extra fallbacks for scrolling vs. non-scrolling, and gets the total
list of fallbacks down to three. With this change, the picker will
show up in one of four positions (above/below and right/left) either
with or without scrollbars depending on available space. The
`position-try-order: most-block-size` property is also added so that
the fallback with the most space for options gets selected.

Note: while working on this, I also discovered two tests that had
implicit dependencies on the number of supported fallback positions,
assuming it was exactly 5. I fixed these tests so that they should
pass even if more than 5 are supported.

Bug: 363205525,362536387
Change-Id: Ica125275d3fcb6e1a3bd32634261084fffd15d82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6071443
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1393860}