The specs say that the second Blob constructor argument is optional (http://www.w3.org/TR/FileAPI/#constructorParams), however Chrome and FF implement that differently. FF checks if the second arg is not undefined, while Chrome checks "arguments.length".

As a result, if the second argument is undefined, Chrome will throw an error (https://chromium.googlesource.com/chromium/blink/+/72fef91ac1ef679207f51def8133b336a6f6588f/LayoutTests/fast/files/blob-constructor.html#55), while FF will just ignore it.

This change makes it so that the second arg is not passed at all when it's undefined, and Chrome is happy as a result.