Remove outdated comment about CSP for <track>

The spec now explictly calls out associated text tracks:
https://dvcs.w3.org/hg/content-security-policy/raw-file/tip/csp-specification.dev.html#media-src

BUG=none
NOTRY=true

Review URL: https://codereview.chromium.org/54683002

git-svn-id: svn://svn.chromium.org/blink/trunk@161057 bbb929c8-8fbe-4397-9dbb-9b2b20218538