In ScrollTree::PullDeltaForMainThread we used to floor the scroll offset which led to  issue 663291  on desktop when blink would draw the main frame using the floored offset but cc would snap the scroll to the nearest pixel (See TransformTree::UpdateSnapping). I resolved this for the common case by rounding instead, but there are cases where this won't be correct.

If there are transforms, or the device scale factor is not 1, rounding may not be the closest scroll offset to the snapped position. We should know the scroll snap in the scroll tree so that we can apply the same snapping when sending the position back to blink.