Whenever the map is resized, the bounds will chage. Therefore, you can use the bounds_changed event:

google.maps.event.addListener(map, 'bounds_changed', function() {

    var bounds = map.getBounds();

});

It has the following description:

This event is fired when the viewport bounds have changed.

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If you only want the event to be called after the map is resized, you can use a boolean variable to track whether the map has just been resized, like so:

var mapResized = false;

Then, whenever you trigger the resize function, you can set the mapResized variable to true. You can do so using a function:

function resizeMap(map) {

    google.maps.event.trigger(map, 'resize');

    mapResized = true;

}

Then, in the bounds_changed event, you can only react to call if mapResized is true, and afterwards set mapResized to false:

google.maps.event.addListener(map, 'bounds_changed', function() {

    if (mapResized) {

        // react here

    }

    mapResized = false;

}