Added an attribute to get the primaryContentShell. r=hyatt

docshell/base/nsIDocShellTreeOwner.idl

@@ -55,6 +55,11 @@ interface nsIDocShellTreeOwner : nsISupports
 	void contentShellAdded(in nsIDocShellTreeItem aContentShell, 
 		in boolean aPrimary, in wstring aID);
 
+	/*
+	Returns the Primary Content Shell
+	*/
+	readonly attribute nsIDocShellTreeItem primaryContentShell;
+
 	/*
 		Tells the implementer of this interface to create a new webBrowserChrome
 		object for it.  Typically this means the implemetor will create a new