Use a precise target type for Beacon requests.

To allow the embedder to distinguish POST requests emanating from
sendBeacon() from those from a <a ping>, assign a more precise target
type to the former.

This is done in preparation for adding Beacon-Age: headers to Beacon
requests.

R=mkwst@chromium.org
BUG=398167

Review URL: https://codereview.chromium.org/468853002

git-svn-id: svn://svn.chromium.org/blink/trunk@180179 bbb929c8-8fbe-4397-9dbb-9b2b20218538