Depth-stencil buffers were using the same pointer for both
attachments, which was causing a double-delete in the
Framebuffer destructor.

BUG=angle:686

Change-Id: I8b263e5a084c33b495ce18ef2a2a810918ef6540
Reviewed-on: https://chromium-review.googlesource.com/205639Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>