Clarify when DocumentLoader's FrameLoader/FrameLoaderClient accessors can be used

third_party/WebKit/Source/core/loader/EmptyClients.cpp

 /*
  * Copyright (C) 2006 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 129 matching lines @@
 
 void EmptyFrameLoaderClient::dispatchWillSendSubmitEvent(HTMLFormElement*) {}
 
 void EmptyFrameLoaderClient::dispatchWillSubmitForm(HTMLFormElement*) {}
 
 DocumentLoader* EmptyFrameLoaderClient::createDocumentLoader(
     LocalFrame* frame,
     const ResourceRequest& request,
     const SubstituteData& substituteData,
     ClientRedirectPolicy clientRedirectPolicy) {
+  DCHECK(frame);
+
   return DocumentLoader::create(frame, request, substituteData,
                                 clientRedirectPolicy);
 }
 
 LocalFrame* EmptyFrameLoaderClient::createFrame(const FrameLoadRequest&,
                                                 const AtomicString&,
                                                 HTMLFrameOwnerElement*) {
   return nullptr;
 }
 
@@ skipping 28 matching lines @@
   return nullptr;
 }
 
 std::unique_ptr<WebApplicationCacheHost>
 EmptyFrameLoaderClient::createApplicationCacheHost(
     WebApplicationCacheHostClient*) {
   return nullptr;
 }
 
 }  // namespace blink