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

crypto/ec_signature_creator.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "crypto/ec_signature_creator.h"
 
+#include "base/logging.h"
 #include "crypto/ec_signature_creator_impl.h"
 
 namespace crypto {
 
 namespace {
 
 ECSignatureCreatorFactory* g_factory_ = NULL;
 
 }  // namespace
 
 // static
 ECSignatureCreator* ECSignatureCreator::Create(ECPrivateKey* key) {
   if (g_factory_)
     return g_factory_->Create(key);
   return new ECSignatureCreatorImpl(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;
 }
 
 }  // namespace crypto