Add navigator.sendBeacon() use counters.

Count API sendBeacon() use, and should its transmission quota be
exceeded, count that too.

R=tkent@chromium.org
BUG=

Review URL: https://codereview.chromium.org/417943005

git-svn-id: svn://svn.chromium.org/blink/trunk@178903 bbb929c8-8fbe-4397-9dbb-9b2b20218538