Improve fidelity of XHR progress events.

When dispatching XMLHttpRequest progress events, for upload as well as
normal/download, supply the required 'loaded' and 'total' properties
on the events, if available, for better reporting to scripts.

Previously just left as 0. The new behavior mirrors what other
implementations are doing.

R=tyoshino,abarth
BUG=318727

Review URL: https://codereview.chromium.org/79953004

git-svn-id: svn://svn.chromium.org/blink/trunk@162689 bbb929c8-8fbe-4397-9dbb-9b2b20218538