Protractor key-down and send keys to zoom browser

Question

I'm attempting to zoom in/out a page using protractor...I've tried the following two methods (I've included both in a helper but no success with either - the current/non-commented out one opens duplicate tabs):

  helpers.zoom_page_out = function zoomOut() {
    //browser.actions().keyDown(protractor.Key.COMMAND).sendKeys('-').perform();
      browser.actions().keyDown(protractor.Key.COMMAND).sendKeys(protractor.Key.SUBTRACT).perform();

   return false;
  };

Any thoughts?