emscripten_async_wget2 progress callback only working on mozilla browsers

For non mozilla browsers the progress percentage has always been zero for me. So I googled and found this:
http://www.w3.org/TR/2014/REC-progress-events-20140211/

I tested my change with IE11, Firefox 29 and Chrome 34 and the percentage values seems to be correct now. I didn't run any tests yet because I could not find any for emscripten_async_wget2. I'm still new to emscripten so I'm not quite ready yet to write a new test myself.
This is my first pull request on github. I hope I didn't do anything wrong.