Summary:
The change enabling virtualization in nested lists contained a hidden assumption that nested lists would only appear within the *cells* of a parent list.
If a list header or footer component contains a `VirtualizedList`, that child list won't be wrapped in a `CellRenderer` component and therefore won't have access to `virtualizedCellRenderer` through its context. This causes an error when the child list tries to access the `cellKey` property on an undefined object.
This change wraps the header/footer views in a `VirtualizedCellWrapper` component which supplies that context properly.
Reviewed By: sahrens
Differential Revision: D6603342
fbshipit-source-id: 4d2d82f04947048a16ec9968121d8ecc8c95655a
a010a0cebd