Add null check of the return value of Document::frame() in DTL::reportResponseReceived().

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

 /*
  * Copyright (C) 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2013, Intel Corporation
  *
  * 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 552 matching lines @@
         // |this| may be dead here in async mode.
         return;
     }
 
     CrossOriginPreflightResultCache::shared().appendEntry(securityOrigin()->toString(), m_actualRequest.url(), preflightResult.release());
 }
 
 void DocumentThreadableLoader::reportResponseReceived(unsigned long identifier, const ResourceResponse& response)
 {
     LocalFrame* frame = document().frame();
+    // We are seeing crashes caused by nullptr (crbug.com/578849). But the frame
+    // must be set here. TODO(horo): Find the root cause of the unset frame.
+    ASSERT(frame);
+    if (!frame)
+        return;
     DocumentLoader* loader = frame->loader().documentLoader();
     TRACE_EVENT_INSTANT1("devtools.timeline", "ResourceReceiveResponse", TRACE_EVENT_SCOPE_THREAD, "data", InspectorReceiveResponseEvent::data(identifier, frame, response));
     InspectorInstrumentation::didReceiveResourceResponse(frame, identifier, loader, response, resource() ? resource()->loader() : 0);
     frame->console().reportResourceResponseReceived(loader, identifier, response);
 }
 
 void DocumentThreadableLoader::handleResponse(unsigned long identifier, const ResourceResponse& response, PassOwnPtr<WebDataConsumerHandle> handle)
 {
     ASSERT(m_client);
 
@@ skipping 306 matching lines @@
     return m_securityOrigin ? m_securityOrigin.get() : document().securityOrigin();
 }
 
 Document& DocumentThreadableLoader::document() const
 {
     ASSERT(m_document);
     return *m_document;
 }
 
 } // namespace blink