[css-font-loading] FontFaceSetLoadEvent fontfaces attribute is illegal WebIDL

FontFaceSetLoadEvent is defined as
<https://drafts.csswg.org/css-font-loading/#dom-fontfacesetloadevent-fontfaces>
:

interface FontFaceSetLoadEvent : Event {
  readonly attribute sequence<FontFace> fontfaces;
};

But WebIDL says <https://heycam.github.io/webidl/#idl-sequence>:

Sequences must not be used as the type of an attribute or constant.
> Note: This restriction exists so that it is clear to specification writers
> and API users that sequences are copied rather than having references to
> them passed around. Instead of a writable attribute of a sequence type, it
> is suggested that a pair of operations to get and set the sequence is used.


I based TouchEvent constructors
<https://github.com/w3c/touch-events/pull/16#issuecomment-119785062>
partially on the design here, but that's causing some concern
<https://github.com/w3c/touch-events/issues/54>.

Thanks,
   Rick