LibWeb: Generate binding for HTMLObjectElement.contentWindow attribute

This only required adding the appropriate definition to the IDL file,
as `NavigableContainer` already implements the logic that we need.

Tests/LibWeb/Text/expected/HTML/HTMLObjectElement-contentWindow.txt

@@ -0,0 +1,2 @@
+object.contentWindow initial value should be null: true
+contentWindow.name should be the same as object.name PASS