TL/DR: You can't use codebender to flash USB Arduinos from a Chromebook. However you may be able to flash an ethernet Arduino.
"chromebook doesn’t support NPapi plugins(aka you can’t use the USB flashing browser plugin). On the other hand you can write and compile code as usual and flashing through the network an Arduino Ethernet with Ariadne is also supported."
http://codebender.uservoice.com/forums/165703-general/suggestions/3386288-make-it-work-on-chromebook
The previous answer does give a way to make your -hardware- flash the USB Arduino, but you have to install Linux on your hardware.