`GraphView` - Update to use ContentArea vs. `ScrollOffset`

[tig]

Once #3820 is done, which fixes

• Create new ScrollBar based on a new Scroll and remove ScrollBarView/ScrollView #2489

we will want to upgrade GraphView to support ScrollBar to use the virtual content scrolling now built into View via Viewport, ViewportSettings, and SetContentSize().

It should be as easy as:

1. Nuke ScrollOffset. Where it's used, use Viewport.X/Y instead.
2. In OnDrawingContent change Move(0,0) to Move(Viewport.X, Viewport.Y) etc...