SpeechRecognitionError use of enum instead of const

My understanding is that the use of const is discouraged in favor of
strings or enumerations: http://dev.w3.org/2006/webapi/WebIDL/#idl-constants

Based on this, I propose changing our SpeechRecognitionError from "const"
to the following "enum" (no change to the corresponding definitions). If
there's no disagreement, I'll update the spec with this on Monday.

    interface SpeechRecognitionError : Event {
        enum ErrorCode {
          "other",
          "no-speech",
          "aborted",
          "audio-capture",
          "network",
          "not-allowed",
          "service-not-allowed",
          "bad-grammar",
          "language-not-supported"
        };

        readonly attribute ErrorCode error;
        readonly attribute DOMString message;
    };

/Glen Shires