[css-content] Should content: url(broken) display the broken image icon?

The following test-case:

```html
<!doctype html>
<div style="content: url(broken)"></div>
```

Shows a broken image icon in WebKit but not Blink. I'm implementing Blink's behavior in https://bugzilla.mozilla.org/show_bug.cgi?id=215083, but may be worth dropping a note or something.