Support FormData on WorkerGlobalScope.

The spec defines FormData as being provided in worker contexts,

  http://www.w3.org/TR/XMLHttpRequest/#interface-formdata
  http://xhr.spec.whatwg.org/#interface-formdata

Follow suit and provide an argument-less FormData constructor and the
interface object on WorkerGlobalScope.

R=arv@chromium.org,nbarth@chromium.org
BUG=360546

Review URL: https://codereview.chromium.org/227483008

git-svn-id: svn://svn.chromium.org/blink/trunk@171294 bbb929c8-8fbe-4397-9dbb-9b2b20218538