Fix issue 299 MutationObserver for markers in Shadow DOM

[mlisook]

In issue #299 it is observed that changes to the google-map-marker collection are not picked up by the google-map under Shadow dom as they are under Shady dom. The code uses a MutationObserver to look for changes, but the content is inside an iron-selector:

<iron-selector id="selector" multi="[[!singleInfoWindow]]" selected-attribute="open" activate-event="google-map-marker-open" on-google-map-marker-close="_deselectMarker">
      <content id="markers" select="google-map-marker"></content>
    </iron-selector>
    <content id="objects" select="*"></content>

this._mutationObserver = new MutationObserver(this._updateMarkers.bind(this));
this._mutationObserver.observe(this.$.selector, {
  childList: true
});

That makes content#markers light dom to the iron-selector, but shadow/shady dom to google-map. Mutation events do not bubble to the outside from within shadow dom. There is even a test for this in the Chromium project - LayoutTests/fast/dom/MutationObserver/shadow-dom.html.

In any case, iron-selector is already observing changes to content#markers. In this PR the google-map is modified to listen for changes to the items property of iron-selector#selector instead of using a MutationObserver.

The existing test, markers-add-remove.html, illustrates both the issue and this fix. With the fix we can add the dom=shadow variety of this test to the list of suites to run.

[ebidel]

let's rename this property to something more appropriate: this._markersChildrenListener

[ebidel]

Nice change. Just the name suggestion.

[mlisook]

I made the name change as suggested to _markersChildrenListener.

[ebidel]

LGTM