views: only show menu scroll controls when they do something

Currently, if the menu items don't all fit in the menu, both scroll
controls always show. This change instead has the scroll controls
each individually show/hide when they can do something - so for
example, when scrolled all the way to the top, the top scroll control
doesn't show, and when scrolled all the way to the bottom, the bottom
scroll control doesn't show.

I made an attempt at tests for this, but got mired in MenuController
and MenuDelegate setup code fairly deeply and decided the complexity
of the tests was not commeasurate with the risks of breakage, so I
removed them.

Bug: 906067
Change-Id: Ia638f139705d8f0af77624c9e686c9776f796fef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2791854
Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#867624}