Avoid exporting label placement property when the chart is 3D.

MS Office has trouble loading the file if you do.  There is an exception,
however.  A pie chart allows label placement option even when 3D.  There
may be other chart types that allow variable label placement when 3D.

Change-Id: I6a9247041ca6ee3ae1b9c245f5919fcb35951f24
Add test to ensure that we don't export label position attributes.

At least for 3D bar chart, exporting this would choke MS Office.

Change-Id: Ic6747f3c5502495d8cae734290183ff89b14fc70