expandable grid does not update scroll bar when a row is expanded

When expanding a row to reveal a sub-grid, the scroll bar does not immediately update. It only updates after it is used to scroll up or down. This issue is problematic when expanding the last couple rows in the grid, you must first scroll up to reset the scroll bar, then scroll back down.

How to reproduce:

1. Go to tutorial for expandable grid: http://ui-grid.info/docs/#/tutorial/306_expandable_grid
2. Scroll to the bottom of the top grid and expand row 499
3. Notice that the scroll bar does not adjust, and you must scroll up before being able to scroll down