Add UseCounter for synchronous XMLHttpRequests

This patch adds a UseCounter for only synchronous XMLHttpRequests.
This will count exactly once per per page using sync XHR requests.

Review URL: https://codereview.chromium.org/337283003

git-svn-id: svn://svn.chromium.org/blink/trunk@176351 bbb929c8-8fbe-4397-9dbb-9b2b20218538