Add Date.now shim to wait-for-cond-in-browser for non-ES5 browsers

Was getting errors in older IE browsers when using waitForConditionInBrowser. I realized it's because the wait-for-cond-in-browser.js file is executed in the browser and it uses Date.now which is not supported in those older IEs. This just adds a quick shim for that if it's not present, from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/now