Add CHECKs to investigate ResourceThrottle crash.

content/browser/loader/throttling_resource_handler.h

 // Copyright (c) 2012 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 CONTENT_BROWSER_LOADER_THROTTLING_RESOURCE_HANDLER_H_
 #define CONTENT_BROWSER_LOADER_THROTTLING_RESOURCE_HANDLER_H_
 
 #include <stddef.h>
 
 #include "base/memory/ref_counted.h"
@@ skipping 53 matching lines @@
   DeferredStage deferred_stage_;
 
   ScopedVector<ResourceThrottle> throttles_;
   size_t next_index_;
 
   GURL deferred_url_;
   net::RedirectInfo deferred_redirect_;
   scoped_refptr<ResourceResponse> deferred_response_;
 
   bool cancelled_by_resource_throttle_;
+
+  // True if currently calling into a throttle, false otherwise.  False when
+  // waiting on a throttle that has delayed the request.
+  // TODO(mmenke):  Remove once https://crbug.com/640545 is resolved.
+  bool currently_calling_throttle_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_LOADER_THROTTLING_RESOURCE_HANDLER_H_