Specifically HTMLIFrameElement and HTMLObjectElement. HTMLEmbedElement
will gain it automatically once it's also converted to inherit from
BrowsingContextContainer.
There are a long list of conditions under which the HTMLObjectElement is
to queue an element task to load / determine an object's representation.
This handles the case where the data attribute has changed.
Much of the spec for determining the object's representation is not
implemented here. Namely, anything to do with XML documents or browser
plugins are left as FIXMEs.
