Enable XHR upload progress events for Workers.

The request flag recording progress event reporting wasn't been copied
out of the worker when creating the loader on the main thread. Do so
to have upload progress events be reported over XMLHttpRequest inside
a Worker.

R=
BUG=173658

Review URL: https://codereview.chromium.org/71433002

git-svn-id: svn://svn.chromium.org/blink/trunk@161923 bbb929c8-8fbe-4397-9dbb-9b2b20218538