fix(wtf-call-tracing): prevent Chrome from making /null requests

The latest version (2014.1.23.1) of the Chrome extension breaks on all apps I've tried because during instrumentation all script tags are transformed into script tags that load scripts from `{base}/null` url.

So given:

```html
<script src="myScript.js"></script>
```

I end up with

```
<script src="null">
  // instrumented contents of myScript.js
  // ...
</script>
```

Because the `src` attribute is still present on the tag when the browser processes it, the browser will ignore the script body and try to make request to "/null" url.