3.0-controls-scrum-70: memory leak in TreeView

There are following regressions in build 3.0-controls-scrum-70:
 TreeView-Expand-adhoc-items100-depth: -29% (-56.78)
 TreeView-Expand-adhoc-items100-wide -21% (-50.22)

 NodeMemory.TreeItem-rendering-on-48mb: -22% (-15,650.00)

These regressions are caused by memory leak in TreeViewSckin. I will provide more details in next comment.

Steps to reproduce:
> compile attached TreeViewExpandPerfTest.java
> java -Xmx128m -verbose:gc -XX:+HeapDumpOnOutOfMemoryError -cp "..." TreeViewExpandPerfTest

The memory leak first appeared in build 3.0-controls-scrum-70
which contains only following change:
   1. RT-20616: TreeView creates new cells every time root is changed — jgiles / detail