PlzNavigate: Refactor unit test helper functions

content/browser/loader/navigation_url_loader.cc

 // Copyright 2014 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 "content/browser/loader/navigation_url_loader.h"
 
 #include "content/browser/frame_host/navigation_request_info.h"
 #include "content/browser/loader/navigation_url_loader_factory.h"
 #include "content/browser/loader/navigation_url_loader_impl.h"
 
@@ skipping 13 matching lines @@
                                    common_params, request_info.Pass(),
                                    request_body, delegate);
   }
   return scoped_ptr<NavigationURLLoader>(new NavigationURLLoaderImpl(
       browser_context, frame_tree_node_id, common_params, request_info.Pass(),
       request_body, delegate));
 }
 
 void NavigationURLLoader::SetFactoryForTesting(
     NavigationURLLoaderFactory* factory) {
+  DCHECK(g_factory == nullptr || factory == nullptr);
   g_factory = factory;
 }
 
 }  // namespace content