Introduce wtf Deque::operator[].

R=jochen@chromium.org, jamesr@chromium.org, erik.corry@gmail.com

Review URL: https://codereview.chromium.org/527583002

git-svn-id: svn://svn.chromium.org/blink/trunk@181192 bbb929c8-8fbe-4397-9dbb-9b2b20218538