Re-land patch for bug 83536, merging principal objects.

Also includes fixes from bug 216041.
r=bzbarsky
sr=jst

content/base/src/nsDOMSerializer.cpp

@@ -50,7 +50,6 @@
 
 #include "nsIJSContextStack.h"
 #include "nsIScriptSecurityManager.h"
-#include "nsICodebasePrincipal.h"
 #include "nsIURI.h"
 
 nsDOMSerializer::nsDOMSerializer()
@@ -156,11 +155,8 @@ nsresult CheckSameOrigin(nsIDOMNode *aRoot)
 
       doc->GetPrincipal(getter_AddRefs(principal));
 
-      nsCOMPtr<nsICodebasePrincipal> codebase_principal =
-        do_QueryInterface(principal);
-
-      if (codebase_principal) {
-        codebase_principal->GetURI(getter_AddRefs(root_uri));
+      if (principal) {
+        principal->GetURI(getter_AddRefs(root_uri));
       }
 
       if (root_uri) {