Relying on the implicit conversion by `::FromContent` doesn't work for
us because we inherit that from `nsGenericHTMLElement` and isn't specific
to `HTMLAreaElement`, so it would accept other elements.
This also updates the Attribute changed event handler to exclude `<a>`
elements and cleans up an unused variable from the old code.
This was part of the HTML4 standard but only ever implemented by Mozilla
and removed again in HTML5 due to lack of adoption. Retaining this could
cause web compat issues and unspecified behavior in the layout engine.
Moonchild
Eitan Isaacson
Matt A. Tobin