Remove activeSourceCount and playbackState from WebAudio

These attributes are no longer part of the WebAudio specification and
should be removed.

BUG=355798

Review URL: https://codereview.chromium.org/210683002

git-svn-id: svn://svn.chromium.org/blink/trunk@171299 bbb929c8-8fbe-4397-9dbb-9b2b20218538