Fix server side rendering

[Horat1us]

Usage of window breaks server side rendering

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[Horat1us]

I signed it!

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CLAs look good, thanks!

[Remo]

@Horat1us google doesn't maintain this repo anymore. We've been fixing a few thing in this repo https://github.com/gmaps-marker-clusterer/gmaps-marker-clusterer
I also tried to make a similar change, but unfortunately I broke a few things gmaps-marker-clusterer/gmaps-marker-clusterer#15