Fix leaflet-layer-group

[Compufreak345]

Fixed a typo in a method name, added a semicolon, added content-tag so this.children actually finds children.

[Compufreak345]

Hmm I just saw that for dynamically added (dom-repeat) items this only works in Chrome right now, in Firefox & Safari the elements inside the group do not get drawn if added dynamically. Static items work. I'll have a deeper look into it tomorrow. Probably has something to do with the MutationObserver.

[Compufreak345]

I fixed the problem mentioned in my last comment by re-adding the mutation observer like mentioned in http://stackoverflow.com/questions/31156079/how-to-use-mutation-observers-in-polymer-1-0/31571790#31571790 . There also was an error loading the map in Safari caused by trying to detach a non-existing mutation observer in leaflet-core. I just checked if it exists and did not see any negative side effects, but it's probably not the most elegant solution.