Update XMLHttpRequest use counter to count workers

Move UseCounter to createRequest() and use executionContext
instead of document to count XHR sync requests from workers once
http://code.google.com/p/chromium/issues/detail?id=376039 is fixed.

Review URL: https://codereview.chromium.org/342703002

git-svn-id: svn://svn.chromium.org/blink/trunk@176448 bbb929c8-8fbe-4397-9dbb-9b2b20218538