Fix NPE and scroll behavior in TabSheet (#14348).

After removing a tab, the TabSheet now scrolls when it is necessary to
show the remaining tabs. A test (TabSheetScrollOnTabCloseTest) ensures
that no scrolling occurs when the removed tab is not in the visible
area.

Change-Id: I81e4e504167ec4d0a527e6bfe94dba8b29fb26bc