Demonstrate that Web Audio does not keep callbacks alive long enough.

This is discussed on Bug 82975. Unfortunately, not keeping the
callback alive long enough encourages authors to implement a
workaround which is prone to leaks by retaining (and typically
leaking) a reference to the callback themselves.

BUG=82795
R=crogers@google.com

Review URL: https://codereview.chromium.org/21216002

git-svn-id: svn://svn.chromium.org/blink/trunk@155194 bbb929c8-8fbe-4397-9dbb-9b2b20218538