Use duck typing instead of instanceof Node

instanceof Node doesn't work with nodes generated by the iframe hack
for some reason. Instead, use duck typing by checking for a .cloneNode
method.

>>> ve.createDocumentFromHTML('<body><tt>Foo</tt></body>').body.childNodes[0]
<tt>Foo</tt>
>>> ve.createDocumentFromHTML('<body><tt>Foo</tt></body>').body.childNodes[0] instanceof Node
false

Change-Id: I1ea1253bd204d1070cd01b666b8a90f1cb7e5e14
Merge "Use duck typing instead of instanceof Node"