Added a readonly attribute to get the PresShell. (Not Part of the build)

docshell/base/nsIDocShell.idl

@@ -38,6 +38,7 @@
  */
 
 [ptr] native nsIPresContext(nsIPresContext);
+[ptr] native nsIPresShell(nsIPresShell);
 [ptr] native nsIDocumentLoaderObserver(nsIDocumentLoaderObserver);
 [ptr] native nsIContentViewer(nsIContentViewer);
 
@@ -104,6 +105,11 @@ interface nsIDocShell : nsISupports
 	*/
 	readonly attribute nsIPresContext presContext;
 
+	/*
+	Presentation context for the currently loaded document.  This may be null.
+	*/
+	readonly attribute nsIPresShell presShell;
+
 	/*
 	Content Viewer that is currently loaded for this DocShell.  This may change
 	as the underlying content changes.