Use Resize Observer API for ScrollBar

[kyoshino]

We have removed the non-standard MozScrolledAreaChanged event in #39 to make the ScrollBar widget e10s-friendly. The current implementation using requestAnimationFrame is much better, but it eats memory. The Resize Observer API, that will be implemented in Firefox soon, is the final answer for this.

[kyoshino]

We perhaps don't need the new API. See CSS deep-dive: matrix3d() for a frame-perfect custom scrollbar for an interesting approach.