Support both prefixed and unprefixed AudioContext and OfflineAudioContext.

Per the discussion at https://groups.google.com/a/chromium.org/d/msg/blink-dev/LKKoAVKcqYo/3FuTOErjjZgJ add back support for prefixed AudioContext and OfflineAudioContext.

BUG=

Review URL: https://codereview.chromium.org/259103002

git-svn-id: svn://svn.chromium.org/blink/trunk@173032 bbb929c8-8fbe-4397-9dbb-9b2b20218538