DBW: improvements to event listener audits

[ebidel]

#830 added an audit to catch non-passive event listeners at the page level ( on document, window, and document.body).

We should expand this audit to look for non-passive listeners on other parts of the app. For example, users use touch events on other parts of the DOM (large scrollable regions of the page). These would also benefit from the perf boost.

Chrome is also considering a browser intervention for making all document-level listeners passive by default. That would make the current audit less useful.

• audit passive listeners on other parts of the DOM
• audit mutation events on other parts of the DOM