Add virtual destructor to please one of our compilers.
Add virtual destructor to please one of our compilers.

Change-Id: I931d130caa75ab90b677e14f1a2d0c438c43ed4f

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst