Bump the wait for scrollend / scroll completion timeout for tests that include a smooth scroll

[dlrobertson]

Test List

• scrollend-event-fired-for-mandatory-snap-point-after-load.html
• scrollend-event-fired-for-programmatic-scroll.html?include=root-scrollBy-smooth
• scrollend-event-fired-for-programmatic-scroll.html?include=root-scrollTo-smooth
• scrollend-event-fired-for-programmatic-scroll.html?include=subframe-scrollBy-smooth
• scrollend-event-fired-for-programmatic-scroll.html?include=subframe-scrollTo-smooth
• scrollend-event-fired-for-scroll-attr-change.html?include=root-scrollLeft-smooth
• scrollend-event-fired-for-scroll-attr-change.html?include=root-scrollTop-smooth
• scrollend-event-fired-for-scroll-attr-change.html?include=subframe-scrollLeft-smooth
• scrollend-event-fired-for-scroll-attr-change.html?include=subframe-scrollTop-smooth
• scrollend-event-handler-content-attributes.html
• scrollend-with-snap-on-fractional-offset.html

Rationale

Many scrollend tests either use waitFor or createScrollendPromiseForTarget. The function waitFor has a hard timeout of 700 frames here, while createScrollendPromiseForTarget has a default timeout of 500ms. We should consider bumping these timeout values. Especially in cases where the test makes use of a smooth scroll.

[zcorpan]

The comment above waitFor says

// TODO(crbug.com/1400399): deprecate. We should not use frame based waits in
// WPT as frame rates may vary greatly in different testing environments.

At 60fps, 700 frames is 11.67s. At 120fps, it's 5.83s.

createScrollendPromiseForTarget accepts a timeout argument, which flows through to t.step_timeout() (docs). Could the smooth scroll tests set a longer timeout?

What is the longest duration for a smooth scroll in implementations?