Support shadow DOM across all audits

Some audits don't handle elements inside shadow DOM either gracefully or at all. This issue tracks improvements to those various audits.

- [x] [rel=noopener gatherer]( https://github.com/GoogleChrome/lighthouse/blob/91ad83a37c25e213a6ef8e9166e86bdc6fc5dbdd/lighthouse-core/gather/gatherers/dobetterweb/anchors-with-no-rel-noopener.js#L28)
- [x] axe support for shadow DOM
- [ ] add support for shadow DOM to [accessibility gatherer `getNodePath`](https://github.com/GoogleChrome/lighthouse/blob/a41b5727205d9c1c57bb6c9b1214cbc796a5be24/lighthouse-core/lib/page-functions.js#L171)