Add CHECKs to the ResourceHandler derived classes to see which is returning a NULL IOBuffer:data_.

chrome/browser/renderer_host/async_resource_handler.h

 // Copyright (c) 2006-2008 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.
 
 #ifndef CHROME_BROWSER_RENDERER_HOST_ASYNC_RESOURCE_HANDLER_H_
 #define CHROME_BROWSER_RENDERER_HOST_ASYNC_RESOURCE_HANDLER_H_
 
 #include <string>
 
 #include "base/process.h"
@@ skipping 20 matching lines @@
   bool OnWillRead(int request_id, net::IOBuffer** buf, int* buf_size,
                   int min_size);
   bool OnReadCompleted(int request_id, int* bytes_read);
   bool OnResponseCompleted(int request_id,
                            const URLRequestStatus& status,
                            const std::string& security_info);
 
   static void GlobalCleanup();
 
  private:
-  // When reading, we don't know if we are going to get EOF (0 bytes read), so
-  // we typically have a buffer that we allocated but did not use.  We keep
-  // this buffer around for the next read as a small optimization.
-  static SharedIOBuffer* spare_read_buffer_;
-
   scoped_refptr<SharedIOBuffer> read_buffer_;
   ResourceDispatcherHost::Receiver* receiver_;
   int process_id_;
   int routing_id_;
   base::ProcessHandle process_handle_;
   ResourceDispatcherHost* rdh_;
 
   DISALLOW_COPY_AND_ASSIGN(AsyncResourceHandler);
 };
 
 #endif  // CHROME_BROWSER_RENDERER_HOST_ASYNC_RESOURCE_HANDLER_H_