nsIContentViewer Interface

 Clears the current history entry.  This is used if we need to clear out
 the saved presentation state.

void clearHistoryEntry()

 All users of a content viewer are responsible for calling both
 close() and destroy(), in that order. 

 close() should be called when the load of a new page for the next
 content viewer begins, and destroy() should be called when the next
 content viewer replaces this one.

 |historyEntry| sets the session history entry for the content viewer.  If
 this is null, then Destroy() will be called on the document by close().
 If it is non-null, the document will not be destroyed, and the following
 actions will happen when destroy() is called (*):
  - Sanitize() will be called on the viewer's document
  - The content viewer will set the contentViewer property on the
    history entry, and release its reference (ownership reversal).
  - hide() will be called, and no further destruction will happen.

  (*) unless the document is currently being printed, in which case
      it will never be saved in session history.

void close(in nsISHEntry historyEntry)

 Attach the content viewer to its DOM window and docshell.
 @param aState A state object that might be useful in attaching the DOM
               window.

void open(in nsISupports aState)