I believe AboutURLConnection and its corresponding Handler are effectively unused. Webkit seems to handle about: URLs internally, by creating an empty document, which is the same thing that AboutURLConnection does.

The only reason we need the handler is so that URLs.newURL() can create a URL object for an about: URL spec. The handler doesn't seem to be called afterwards. The first naive attempt to remove AboutURLConnection (see RT-32097) failed because newURL() call sites were not updated.