Add a UseCounter for <a ping> usage.

Chromium and Safari are the only browsers supporting this attribute, and an
imperative version of this functionality is upcoming as the Beacon API.
Mozilla stated they'd prefer to support only one of them, and are erring in
the direction of the Beacon API. Preemptively start getting some data.

NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/183093003

git-svn-id: svn://svn.chromium.org/blink/trunk@168067 bbb929c8-8fbe-4397-9dbb-9b2b20218538