message: ProgressBar.disabled prop is a boolean
  Change-type: patch
diff: active = "main.state.isFlashing()"
  percentage="main.state.getFlashState().percentage"
  label="flash.getProgressButtonLabel()"
  - disabled="main.state.getLastFlashErrorCode() || main.shouldFlashStepBeDisabled()"
  + disabled="!!main.state.getLastFlashErrorCode() || main.shouldFlashStepBeDisabled()"
  callback="flash.flashImageToDrive" >
  </progress-button>
  