A mixin which adds the
PerformanceWatchdog widget to the Viewer widget.
Rather than being called directly, this function is normally passed as
a parameter to Viewer#extend, as shown in the example below.Example:
const viewer = new Cesium.Viewer('cesiumContainer');
viewer.extend(Cesium.viewerPerformanceWatchdogMixin, {
lowFrameRateMessage : 'Why is this going so <em>slowly</em>?'
});