Explicit response to destructor messages before the destructor is actually called

I have two protocols:

protocol NPP
{
  manages NPBrowserStream;

child:
  NPBrowserStream(...);

both:
  ~NPBrowserStream();
};

protocol NPBrowserStream
{
  manager NPP;
  ...
};

The two-way ~NPBrowserStream destructor models two different calls in NPAPI:

NPP_DestroyStream -- the browser closes the stream
NPN_DestroyStream -- the plugin closes the stream

I'm writing bits of NPPInstanceParent right now, and I don't think I can
respond to these messages differently the way I need to:

* If I'm receving NPBrowserStreamDestructor from the other side, I need to
call NPN_DestroyStream
* If I'm sending NPBrowserStreamDestructor, I'm being called from
NPP_DestroyStream

But as far as I can tell there's no
NPPInstanceProtocolParent::RecvNPBrowserStreamDestructor method I can
override... I think I want one before the IPDL machinery calls
NPBrowserStreamDestructor directly.

--BDS