Add CHECKs to investigate ResourceThrottle crash.

content/browser/loader/throttling_resource_handler.h

Index: content/browser/loader/throttling_resource_handler.h
diff --git a/content/browser/loader/throttling_resource_handler.h b/content/browser/loader/throttling_resource_handler.h
index 434151506ad250837132b7824b8bd5b704f7f59b..873aad0dd9c202cfc6783abcaaf856be22415c2d 100644
--- a/content/browser/loader/throttling_resource_handler.h
+++ b/content/browser/loader/throttling_resource_handler.h
@@ -71,6 +71,11 @@ class ThrottlingResourceHandler : public LayeredResourceHandler,
   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