Currently we send actors representing entries over IPC. On the parent side the actor holds the real nsSHEntry (which is stored in the nsSHistory) and on the child side the actor is the entry that we put in the docshell's mLSHE and mOSHE. The docshell shouldn't hold the entry actor directly, but hold structures with the data for the mLSHE and mOSHE. Because moving everything over to this new structure will take time we'll start by just storing a pointer to the entry actor in the structure. We can then gradually move over the data from entry to structure.
Conceptually, messages from parent to child should set the mLSHE to the new state in the structure that we send over IPC and messages from child to parent should signal that the mLSHE is now the mOSHE.

This has been partially implemented by bug 1570255.