Fix JS error on Firefox hidden iframes: verify that getComputedStyle is not returning null before using it

On Firefox, `getComputedStyle` returns `null` if it's called inside a hidden iframe. You can see an example here:
https://www.groupon.com/deals/butter

Open that URL on Firefox. You'll see several JS errors in the console, the first being `TypeError: window.getComputedStyle(...) is null`. This is happening not on that page, but on a hidden iframe on that page (that iframe is loaded on page load but it's only shown when a user takes certain action, so it's a completely valid use case).

So, basically, this code will break:
```javascript
var result = false;
if(window.getComputedStyle) {
  result = window.getComputedStyle(element, null).getPropertyValue('font-size') === '10px';
}
return result;
```

The solution is to first check for the returned value, like this:
```javascript
var result;
var style;
if(window.getComputedStyle && (style = window.getComputedStyle(element, null)) !== null) {
  result = style.getPropertyValue('font-size') === '10px';
}
return result;
```

Different files have different coding styles, so I tried to preserve the coding style for each file.