Implement navigator.sendBeacon()

This provides support for the Beacon API,

 https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/Beacon/Overview.html

allowing async transmission of data using navigator.sendBeacon().

R=abarth@chromium.org,mkwst@chromium.org
BUG=360603

Review URL: https://codereview.chromium.org/232053005

git-svn-id: svn://svn.chromium.org/blink/trunk@173837 bbb929c8-8fbe-4397-9dbb-9b2b20218538