Unify the error checking of crypto::Encryptor and add WARN_UNUSED_RESULT to prevent misuse.

net/socket/dns_cert_provenance_checker.cc

Index: net/socket/dns_cert_provenance_checker.cc
diff --git a/net/socket/dns_cert_provenance_checker.cc b/net/socket/dns_cert_provenance_checker.cc
index 04cbce8b5c1057523e77e101453a21e6a535e4e3..f6bdd8262aa5bd1e6142bc2abd90ebc156385141 100644
--- a/net/socket/dns_cert_provenance_checker.cc
+++ b/net/socket/dns_cert_provenance_checker.cc
@@ -312,7 +312,8 @@ std::string DnsCertProvenanceChecker::BuildEncryptedReport(
 
   std::string plaintext(reinterpret_cast<const char*>(p.data()), p.size());
   std::string ciphertext;
-  encryptor.Encrypt(plaintext, &ciphertext);
+  r = encryptor.Encrypt(plaintext, &ciphertext);
+  CHECK(r);
 
   // We use another Pickle object to serialise the 'outer' wrapping of the
   // plaintext.