There are 2 EmbedPrivate.cpp files.
One in mozilla/embedding/browser/gtk/src/
The other at mozilla/embedding/browser/photon/src/


This is the one at gtk/src/

  // get a handle on the navigation object
  mNavigation = do_QueryInterface(webBrowser);

  // Create our session history object and tell the navigation object
  // to use it.  We need to do this before we create the web browser
  // window.
  mSessionHistory = do_CreateInstance(NS_SHISTORY_CONTRACTID);
  mNavigation->SetSessionHistory(mSessionHistory);            <----------------------- LINE 257 --------------------

  // create the window
  mWindow->CreateWindow();



And the one at photon/src/

	// get a handle on the navigation object
	mNavigation = do_QueryInterface(webBrowser);

	// Create our session history object and tell the navigation object
	// to use it.  We need to do this before we create the web browser
	// window.
	mSessionHistory = do_CreateInstance(NS_SHISTORY_CONTRACTID);
	mNavigation->SetSessionHistory(mSessionHistory);

	// create the window
	mWindow->CreateWindow();   <--------- LINE 257 ----------