Remove Attr.isId and Attr.ownerElement.

LayoutTests/fast/dom/Attr/script-tests/access-after-element-destruction.js

 description("Tests that accessing Attr after its Element has been destroyed works without crashing.");
 
 function gc()
 {
     if (window.GCController)
         return GCController.collect();
 
     // Trigger garbage collection indirectly.
     for (var i = 0; i < 100000; i++)
         new String(i);
 }
 
 var element = document.createElement("p");
 element.setAttribute("a", "b");
 var attributes = element.attributes;
 element = null;
 
 gc();
 
 shouldBe("attributes.length", "1");
 shouldBe("attributes[0]", "attributes.item(0)");
 shouldBe("attributes.getNamedItem('a')", "attributes.item(0)");
 
 shouldBe("attributes.item(0).name", "'a'");
 shouldBe("attributes.item(0).value", "'b'");
-shouldBe("attributes.item(0).ownerElement.tagName", "'P'");
 
 attributes.item(0).value = 'c';
 
 shouldBe("attributes.item(0).value", "'c'");
 
 attributes.removeNamedItem('a');
 
 shouldBe("attributes.length", "0");
 
 element = document.createElement("p");
 element.setAttribute("a", "b");
 var attr = element.attributes.item(0);
 element = null;
 
 gc();
 
 shouldBe("attr.name", "'a'");
 shouldBe("attr.value", "'b'");
-shouldBe("attr.ownerElement.tagName", "'P'");
 
 attr.value = 'c';
 
 shouldBe("attr.value", "'c'");