Added new method- `measureAllCells`- to `Grid`, `FlexTable`, and `VirtualScroll` to force-measure all cells.
This supports special use-cases where deferred measuring is not desired.

Added `estimatedRowSize` property to `FlexTable` and `VirtualScroll` to be passed through to the inner `Grid`.

Also added guard to ensure the `onScroll` callback for `Collection`, `Grid`, `FlexTable`, and `VirtualScroll` is never called with a negative number.
