Use items-changed event ILO MutationObserver for markers (issue 299)

[mlisook]

… issue #299 ...

The MutationObserver works in shady dom but not in shadow dom because it is observing a shadow dom element (iron-selector.#selector). Mutation events cannot pierce the shadow dom (there is even a test for this in chromium - MutationObserver / shadow-dom.html ).

Instead, we can use the iron-selector items property and a change listener.
This also allows us to include the markers-add-remove.html test in both shadow (where it previously failed) and shady.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[mlisook]

I signed it!

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for the commit author(s). If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.

[mlisook]

I signed it!