Operating system / user transporter test definition

I have a sequence of mouse / key buttons in a Protractor test that differs depending on the OS (Mac versus everything else). I would like to change which key is held down by the mouse click, depending on the OS or User Agent.

Depending on where I'm testing (local Mac OSx + Chrome or headless PhantomJS on Linux) a command key or Ctrl key is required.

The current test looks like this:

describe('my test', function() {

  //get elements with code not shown
   ptor.actions()
                        .sendKeys(protractor.Key.CONTROL)  //mac protrator.Key.COMMAND non mac protractor.Key.CONTROL
                        .mouseDown(trs[0])
                        .mouseUp(trs[0])
                        .sendKeys(protractor.Key.NULL)
                        .perform();
                    trs[0].evaluate('selectedItems.length')
                        .then(function(count) {
                            expect(count).toBe(0);
                        })

});