Remove invalid comment about HRTFDatabaeLoader being a singleton.

It's not

BUG=none
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/56533003

git-svn-id: svn://svn.chromium.org/blink/trunk@161283 bbb929c8-8fbe-4397-9dbb-9b2b20218538