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

third_party/WebKit/Source/web/FrameLoaderClientImpl.cpp

 /*
  * Copyright (C) 2009, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 662 matching lines @@
   if (WebFrameClient* client = m_webFrame->client())
     client->didMatchCSS(m_webFrame, WebVector<WebString>(addedSelectors),
                         WebVector<WebString>(removedSelectors));
 }
 
 DocumentLoader* FrameLoaderClientImpl::createDocumentLoader(
     LocalFrame* frame,
     const ResourceRequest& request,
     const SubstituteData& data,
     ClientRedirectPolicy clientRedirectPolicy) {
+  DCHECK(frame);
+
   WebDataSourceImpl* ds =
       WebDataSourceImpl::create(frame, request, data, clientRedirectPolicy);
   if (m_webFrame->client())
     m_webFrame->client()->didCreateDataSource(m_webFrame, ds);
   return ds;
 }
 
 String FrameLoaderClientImpl::userAgent() {
   WebString override =
       m_webFrame->client() ? m_webFrame->client()->userAgentOverride() : "";
@@ skipping 297 matching lines @@
 WebDevToolsAgentImpl* FrameLoaderClientImpl::devToolsAgent() {
   return WebLocalFrameImpl::fromFrame(m_webFrame->frame()->localFrameRoot())
       ->devToolsAgentImpl();
 }
 
 KURL FrameLoaderClientImpl::overrideFlashEmbedWithHTML(const KURL& url) {
   return m_webFrame->client()->overrideFlashEmbedWithHTML(WebURL(url));
 }
 
 }  // namespace blink