Auto-hiding scrollbars currently reveal whenever their prepaint geometry
changes. That matches browser behavior, but it causes continuously
updating views to restart the visibility timer on every content change
even when the user has not scrolled.

Add a per-instance `ScrollbarRevealPolicy`. The default
`ScrollOrContentChange` policy preserves the existing behavior for all
current callers. Views with continuously growing content can opt into
`ScrollOnly`, which reveals only when the offset changes independently
of content size, including when a view remains anchored to the bottom.

Tests pin the default behavior, the opt-in behavior, ordinary scrolling,
stationary content growth, bottom-anchored growth, and scrolling while
content is growing.

Release Notes:

- N/A