Mark HTMLMediaElement/MediaController.duration as unrestricted double

This does not change the generated code, it is only to match the spec:
http://whatwg.org/html#htmlmediaelement
http://whatwg.org/html#mediacontroller

BUG=354298

Review URL: https://codereview.chromium.org/266473003

git-svn-id: svn://svn.chromium.org/blink/trunk@172981 bbb929c8-8fbe-4397-9dbb-9b2b20218538