bssl::UniquePtr and FOO_up_ref do not play well together. Add a helper
to simplify this. This allows us to write things like:

   foo->cert = UpRef(bar->cert);

instead of:

   if (bar->cert) {
     X509_up_ref(bar->cert.get());
   }
   foo->cert.reset(bar->cert.get());

This also plays well with PushToStack. To append something to a stack
while taking a reference, it's just:

   PushToStack(certs, UpRef(cert))

Change-Id: I99ae8de22b837588a2d8ffb58f86edc1d03ed46a
Reviewed-on: https://boringssl-review.googlesource.com/29584
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>