Allow static invocation of DOMDocument::load* methods

Summary: It should be possible to statically invoke these loaders to create new DOMDocument
objects.

Fixes #1955

{sync, type="child", parent="internal", parentrevid="1686673", parentrevfbid="1506117346322140", parentdiffid="5688837"}

Reviewed By: @JoelMarcey

Differential Revision: D1686673

Signature: t1:1686673:1416546346:27dadb68e89f406350add7994c14635dc590c26c