Rather than keeping SnapContainerData for all scrollers in a SnapCoordinator attached to the document, it makes more sense to just associate SnapContainerData directly with its ScrollableArea.

For PaintLayerScrollableArea, there is already a PaintLayerScrollableAreaRareData class, which is the obvious place for this.

For RootFrameViewport, it's probably OK to just add it to that class directly.

Majid, please triage.

I believe szager@ is planning to do this after M69 branch which is when we plan to ship snap points.

Assigning to szager@ who wanted to land this as a clean up. M69 is branched so it is safe to do this now.