weather: Add dummy processBack method.

This eliminates the following warning from the app engine when the back softkey
is pressed on the options form.

  genericForm.js:566: TypeError: Object #<Object> has no method 'processBack'
  this.formObjects[this.activeForm].processBack();
  ^
  TypeError: Object #<Object> has no method 'processBack'
  at Object.onBack (genericForm.js:566:36)
  at Input.onsoftkey4 (native)
  at Input.<anonymous> (unknown source)