Pass a URL as a query parameter to the surveys

We need to be able to display destination URLs in the questions. Here's how it works:
1. The background script gets the destination URL as part of the event
2. Background script sanitizes the URL
3. Background script appends the sanitized URL as a query parameter when opening survey.html
4. The survey driver checks that the URL is sanitized
5. The URL can be used in questions (as part of textContent)

Extensions do not have questions about URLs, so this only applies to the non-extension surveys.
This CL also adds test cases for the sanitization method.
Looks like you can refactor lines 21-24 (and 42) into a separate method such as ```function parseKeyValuePair(string, keyName) {}``` which returns the associated value (```keyName``` being the expected key, which is  ```'js'``` here). You seem to be parsing the same way in here and below for ```'url'``` param.