Bug 19239

Currently, there is a bit of XML Entity support, but not enough to be useful. The actual entities are expanded when encountered, which is good. But we also have an Entity DOM class, which is never instantiated, and an EntityReferece DOM class, which is instantiated only by createEntityReference, but never has any children, so is useless. For this bug I will change createEntityReference so it will throw NOT_SUPPORTED_ERR for XML documents, just as it does for HTML documents. I think this will create no practical difficulty, since it does nothing useful today, and is highly unlikely to be widely used on the web. Once this is done, we can guarantee there are no Entity or EntityReference nodes, and hence no read-only nodes, at all in WebKit, so we can remove some relatively costly checks in the code. Some day if we find a reason and want to bring these node types back, we can find an efficient way to do so.