Forgot to add virtual destructors.

BUG=angleproject:1437

This reverts commit a42e8b2c.

Change-Id: If33fecfeca9947deedf4668c64dbadf25a5dc5eb
Reviewed-on: https://chromium-review.googlesource.com/360122Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>