Implement WebIDL implements

Source/core/dom/ParentNode.idl

Index: Source/core/dom/ParentNode.idl
diff --git a/Source/core/dom/ParentNode.idl b/Source/core/dom/ParentNode.idl
new file mode 100644
index 0000000000000000000000000000000000000000..6668c816137853e19acc3e554f8697b8e22a6ecd
--- /dev/null
+++ b/Source/core/dom/ParentNode.idl
@@ -0,0 +1,12 @@
+[
+    NoInterfaceObject
+] interface ParentNode {
+    readonly attribute HTMLCollection children;

[arv (Not doing code reviews), 2013/06/18 15:18:21]

Is there a problem if we have [PerWorldBindings] here?

The Element version of these had [PerWorldBindings] for speed.

+    readonly attribute Element firstElementChild;
+    readonly attribute Element lastElementChild;
+    readonly attribute unsigned long childElementCount;
+};
+
+Document implements ParentNode;
+DocumentFragment implements ParentNode;
+Element implements ParentNode;