Currently, programmatic updates to scroll offset via script plus
methods on Element or DOMWindow force layout before applying the
scroll offset, because otherwise clamping of the scroll offset
cannot be computed.

The code also schedules a new frame, which is when the scroll is really
applied and drawn to the screen.

Instead, we should implement such methods by mutating something like
a PendingScroll object, dirtying scroll/layout state, and requesting
a frame update. then when the new frame is generated, layout will happen
and the PendingScroll will be turned into a real scroll.

This allows developers to write scroll state without forced layouts
outside of frame generation, thus improving performance and becoming
compatible wit efforts to understand and control frame generation
for performance and scalability (async dom, throttling, etc).