Add a DCHECK to ECSignatureCreator::SetFactoryForTesting to help avoid use-after-free problems.

crypto/ec_signature_creator.cc

Index: crypto/ec_signature_creator.cc
diff --git a/crypto/ec_signature_creator.cc b/crypto/ec_signature_creator.cc
index cbaa820cec96001eb4ad79c3f33312db043920ec..a6887bc117b7f5e7206e42d62367a9c3be571ad3 100644
--- a/crypto/ec_signature_creator.cc
+++ b/crypto/ec_signature_creator.cc
@@ -4,6 +4,7 @@
 
 #include "crypto/ec_signature_creator.h"
 
+#include "base/logging.h"
 #include "crypto/ec_signature_creator_impl.h"
 
 namespace crypto {
@@ -24,6 +25,9 @@ ECSignatureCreator* ECSignatureCreator::Create(ECPrivateKey* key) {
 // static
 void ECSignatureCreator::SetFactoryForTesting(
     ECSignatureCreatorFactory* factory) {
+  // We should always clear the factory after each test to avoid
+  // use-after-free problems.
+  DCHECK(!g_factory_ || !factory);
   g_factory_ = factory;
 }