Bug 951467. Give scroll layer items the bounds of the scroll port, not the bounds of the display port. r=roc

The bounds of the scroll port match what will actually be drawn on the screen.

The bounds of the contained content (sized to the display port) are still accessible via mList.GetBounds, and similarly the visible rect of the contained content is mList.GetVisibleRect. The external bounds/visible rect are GetBounds and GetVisibleRect on the nsDisplayScrollLayer object itself.

We implement nsDisplayScrollInfoLayer::GetBounds solely so that it continues to return an empty rect because we expect active empty layers to have empty visible rects.

We no longer have to set our mVisibleRect in nsDisplayScrollLayer::ComputeVisibility because nsDisplayList::ComputeVisibilityForSublist now does it correctly for us (like other items).

We also have to teach ContainerState::ProcessDisplayItems to not set the visible region for scroll layers because it has the external visible region, not the larger internal display port sized visible region. We instead let BuildContainerLayer set the visible region of the layer.