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

chrome/browser/renderer_host/safe_browsing_resource_handler.cc

 // 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.
 
 #include "chrome/browser/renderer_host/safe_browsing_resource_handler.h"
 
+#include "base/logging.h"
 #include "chrome/browser/renderer_host/resource_dispatcher_host.h"
 #include "chrome/browser/renderer_host/resource_message_filter.h"
 #include "chrome/common/notification_service.h"
 #include "net/base/net_errors.h"
+#include "net/base/io_buffer.h"
 
 // Maximum time to wait for a gethash response from the Safe Browsing servers.
 static const int kMaxGetHashMs = 1000;
 
 SafeBrowsingResourceHandler::SafeBrowsingResourceHandler(
     ResourceHandler* handler,
     int render_process_host_id,
     int render_view_id,
     const GURL& url,
     ResourceType::Type resource_type,
@@ skipping 75 matching lines @@
         FROM_HERE,
         NewRunnableMethod(this, &SafeBrowsingResourceHandler::OnGetHashTimeout),
         kMaxGetHashMs);
   }
 
   if (in_safe_browsing_check_ || displaying_blocking_page_) {
     rdh_->PauseRequest(render_process_host_id_, request_id, true);
     paused_request_id_ = request_id;
   }
 
-  return next_handler_->OnWillRead(request_id, buf, buf_size, min_size);
+  bool rv = next_handler_->OnWillRead(request_id, buf, buf_size, min_size);
+  // TODO(willchan): Remove after debugging bug 16371.
+  if (rv)
+    CHECK((*buf)->data());
+  return rv;
 }
 
 bool SafeBrowsingResourceHandler::OnReadCompleted(int request_id,
                                                   int* bytes_read) {
   return next_handler_->OnReadCompleted(request_id, bytes_read);
 }
 
 bool SafeBrowsingResourceHandler::OnResponseCompleted(
     int request_id, const URLRequestStatus& status,
     const std::string& security_info) {
@@ skipping 81 matching lines @@
 void SafeBrowsingResourceHandler::Observe(NotificationType type,
                                           const NotificationSource& source,
                                           const NotificationDetails& details) {
   DCHECK(type.value == NotificationType::RESOURCE_MESSAGE_FILTER_SHUTDOWN);
   if (in_safe_browsing_check_) {
     safe_browsing_->CancelCheck(this);
     in_safe_browsing_check_ = false;
     Release();
   }
 }
-