I understand the concern that a proprietary component may be performing unknown instructions, and indeed Chromium does download hotword-x86-64.nexe on startup, but it has been carefully designed as an opt-in feature. If you do not turn on "Enable "Ok Google" to start a voice search" (in chrome://settings), Chromium will not run the plugin. You do not need to trust Google engineers to tell you this; the open source Chromium code has the logic to decide whether to run the plugin.
I have posted a detailed response (including the link to the place in the Chromium source code where the module gets run) on our bug tracker at http://crbug.com/500922#c6.