A latitude-longitude viewport, represented as two diagonally opposite `low`
and `high` points. A viewport is considered a closed region, i.e. it includes
its boundary. The latitude bounds must range between -90 to 90 degrees
inclusive, and the longitude bounds must range between -180 to 180 degrees
inclusive. Various cases include:
 - If `low` = `high`, the viewport consists of that single point.
 - If `low.longitude` > `high.longitude`, the longitude range is inverted
   (the viewport crosses the 180 degree longitude line).
 - If `low.longitude` = -180 degrees and `high.longitude` = 180 degrees,
   the viewport includes all longitudes.
 - If `low.longitude` = 180 degrees and `high.longitude` = -180 degrees,
   the longitude range is empty.
 - If `low.latitude` > `high.latitude`, the latitude range is empty.
Both `low` and `high` must be populated, and the represented box cannot be
empty (as specified by the definitions above). An empty viewport will result
in an error.
For example, this viewport fully encloses New York City:
{
    "low": {
        "latitude": 40.477398,
        "longitude": -74.259087
    },
    "high": {
        "latitude": 40.91618,
        "longitude": -73.70018
    }
}

Protobuf type google.geo.type.Viewport