This patch:
- simplifies test reusing the `offscreenbuttons.html` asset
- aligns IntersectionObserver usage with the one we have for
`ElementHandle._scrollIntoViewIfNeeded`.
Originally, we use `Element.scrollIntoViewIfNeeded` to make sure
button is on screen before trying to click it.
However, `Element.scrollIntoViewIfNeeded` doesn't work in certain
scenarios, e.g. when element is partially visible and horizontal
scrolling is required to make it fully visible.
This patch polyfills `element.scrollIntoViewIfNeeded` using
IntersectionObserver and `Element.scrollIntoView`.
Fixes#2804.
