Add a comment to a subtle use of OwnPtr::release()

BUG=240653
R=dstockwell@chromium.org, dstockwell

Review URL: https://codereview.chromium.org/25040006

git-svn-id: svn://svn.chromium.org/blink/trunk@158621 bbb929c8-8fbe-4397-9dbb-9b2b20218538