Add tagName to ve.dm.MWGalleryNode

I told James this wasn't needed, but I was wrong. It is null in
the parent class, and DOM throws on document.createElement(null).

Bug: 60307
Change-Id: Iff8fda9acf9f37e537d7d6cf6106a801d6b71338
Merge "Add tagName to ve.dm.MWGalleryNode"