Bug 1368571: Modify handler and interceptor marshaling code so that it strips out any handlers from proxies that are destined for non-Gecko processes; r=jimm

MozReview-Commit-ID: A1lCqvbQYAF

There is no clean API-based solution to this, so instead I went grovelling
through the DCOM wire protocol and was able to write a function that converts
handler OBJREFs into standard OBJREFs.

See also:
https://msdn.microsoft.com/en-us/library/cc226801

ipc/mscom/MainThreadHandoff.cpp

@@ -453,6 +453,7 @@ MainThreadHandoff::GetHandler(NotNull<CLSID*> aHandlerClsid)
   if (!mHandlerProvider) {
     return E_NOTIMPL;
   }
+
   return mHandlerProvider->GetHandler(aHandlerClsid);
 }