Unfortunately, it's not enough to be able to turn it on thanks to the
PURE_VIRTUAL macro. But it gets us most of the way there.
Change-Id: Ie6ad5119fcfd420115fa49d7312f3586890244f4
Reviewed-on: https://boringssl-review.googlesource.com/c/34949
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>