docs(maps): Describe how to get map instance from within component.ts to input to MapMarker, InfoWindow

### Documentation Feedback

Currently the example docs rely on a single use case where the user interacts with the map and a (mapClick) event grabs the map instance and passes it back to a component.ts function. If a developer wishes to decorate a map with a marker or an info window before a user interacts with it, there is no map reference to use.

It appears that standard @ViewChild methods for obtaining a dom reference do not work with the angular google-maps component. We could use a good example of how to get the map reference.

### Affected documentation page

https://github.com/angular/components/tree/master/src/google-maps/map-marker

从 src/google-maps/map-marker 开始，查看现有的文档示例，该示例通过 mapClick 事件获取地图。确定如何在用户交互之前从 component.ts 获取地图引用，并将其传递给 MapMarker 或 InfoWindow。当受影响的文档包含一个在交互之前装饰地图的可运行示例时，即视为完成。

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