Kill attribute order preservation

Per the bug report, it's useless because Firefox does not preserve order
in the .attributes array (but sorts it alphabetically instead), and so we
actually get the same exact behavior regardless of whether we use ordering
cleverness or just a straight-up plain object: order is preserved in Chrome,
and alphabetized in Firefox.

Bug: 48980
Change-Id: I1463d06db4900083dd4c565292bbabef09194b43