Make sure that all tracks created by the WebAudio code are proprly
initialized by making sure they are reported down to the chromium code.

Review URL: https://codereview.chromium.org/218773003

git-svn-id: svn://svn.chromium.org/blink/trunk@170758 bbb929c8-8fbe-4397-9dbb-9b2b20218538