Corresponds to https://github.com/whatwg/html/pull/10904
However, we don't implement most of what is changed in that PR, so this
simply moves the FIXME'd getContextAttributes() method from one place
to another.
From the Web IDL spec: https://heycam.github.io/webidl/#idl-undefined
[...]
undefined constant values in IDL are represented with the `undefined`
token.
[...]
Note: This value was previously spelled `void`, and more limited in how
it was allowed to be used.