VisibleRegion.toUrlValue() return error

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- [ ] question
- [X] any problem or bug report
- [ ] feature request

**If you choose 'problem or bug report', please select OS:** (check one with "x")
- [ ] **Android**
- [ ] **iOS**
- [X] **Browser**

**cordova information:** (run `$> cordova plugin list`)

```
cordova-plugin-device 2.0.2 "Device"
cordova-plugin-googlemaps 2.7.1 "cordova-plugin-googlemaps"
cordova-plugin-ionic-keyboard 2.2.0 "cordova-plugin-ionic-keyboard"
cordova-plugin-ionic-webview 5.0.0 "cordova-plugin-ionic-webview"
cordova-plugin-splashscreen 6.0.2 "Splashscreen"
cordova-plugin-statusbar 2.4.2 "StatusBar"
```

**If you use `@ionic-native/google-maps`, please tell the package.json (only `@ionic-native/core` and `@ionic-native/google-maps` are fine mostly)**

```
"@ionic-native/core": "^5.36.0",
"@ionic-native/google-maps": "^5.5.0",
```

**Current behavior:**

This code:
```typescript
const map = GoogleMaps.create('map-canvas', {
mapType: GoogleMapsMapTypeId.ROADMAP,
camera: {
target: {
lat: -15.800739,
lng: -47.861314
},
zoom: 15
}
});
map.on(GoogleMapsEvent.MAP_READY).subscribe(()=>{
console.log(map.getVisibleRegion().toUrlValue());
});
```

produces this error:

```
ERROR TypeError: this.southwest.toUrlValue is not a function
at LatLngBounds.toUrlValue (LatLngBounds.js:36:31)
at SafeSubscriber._next (map.service.ts:131:42)
```

**Expected behavior:**
Expected to get "[lat_sw,lng_sw,lat_ne,lng_ne]" on the terminal

Inspecting where the code fails:
```typescript
LatLngBounds.prototype.toUrlValue = function (precision) {
precision = precision || 6;
return '[' + this.southwest.toUrlValue(precision) + ',' + this.northeast.toUrlValue(precision) + ']';
};
```

It seems that the function expects that this.southwest and this.northeast are of type LatLng which implements toUrlValue().

Howerver, VisibleRegion class defines both as ILatLng which does not implement toUrlValue().

Workaround: convert VisibleRegion.southwest and VisibleRegion.northeast to LatLng
```typescript
const map = GoogleMaps.create('map-canvas', {
mapType: GoogleMapsMapTypeId.ROADMAP,
camera: {
target: {
lat: -15.800739,
lng: -47.861314
},
zoom: 15
}
});
map.on(GoogleMapsEvent.MAP_READY).subscribe(()=>{
const visibleRegion = map.getVisibleRegion();
//CONVERT SOUTHWEST AND NORTHEAST TO LATLNG
const southwest = visibleRegion.southwest;
const northeast = visibleRegion.northeast;
visibleRegion.southwest = new LatLng(southwest.lat,southwest.lng);
visibleRegion.northeast = new LatLng(northeast.lat,northeast.lng);
console.log(visibleRegion.toUrlValue());
});
```
Result:
```
[-15.812301,-47.869897,-15.789176,-47.852731]
```

Start at LatLngBounds.toUrlValue() and inspect the VisibleRegion, ILatLng, and LatLng definitions to confirm the type mismatch described in the report. Reproduce the browser example and verify that calling toUrlValue() directly on the visible region returns the expected southwest and northeast coordinate string without the workaround.

索引モデルが issue の本文から書いたものです。