Deprecate XHR progress event properties 'position' and 'totalSize'.

Use the properties 'loaded' and 'total' respectively instead.

R=tyoshino@chromium.org, haraken@chromium.org
BUG=357112

Review URL: https://codereview.chromium.org/492213004

git-svn-id: svn://svn.chromium.org/blink/trunk@180775 bbb929c8-8fbe-4397-9dbb-9b2b20218538