function viewerPerformanceWatchdogMixin(viewer, options) {
  //>>includeStart('debug', pragmas.debug);
  if (!defined(viewer)) {
    throw new DeveloperError("viewer is required.");
  }
  //>>includeEnd('debug');

  options = options ?? Frozen.EMPTY_OBJECT;

  const performanceWatchdog = new PerformanceWatchdog({
    scene: viewer.scene,
    container: viewer.bottomContainer,
    lowFrameRateMessage: options.lowFrameRateMessage,
  });

  Object.defineProperties(viewer, {
    performanceWatchdog: {
      get: function () {
        return performanceWatchdog;
      },
    },
  });
}
A mixin which adds the {@link PerformanceWatchdog} widget to the {@link Viewer} widget.
Rather than being called directly, this function is normally passed as
a parameter to {@link Viewer#extend}, as shown in the example below.
@function

@param {Viewer} viewer The viewer instance.
@param {object} [options] An object with properties.
@param {string} [options.lowFrameRateMessage='This application appears to be performing poorly on your system.  Please try using a different web browser or updating your video drivers.'] The
       message to display when a low frame rate is detected.  The message is interpeted as HTML, so make sure
       it comes from a trusted source so that your application is not vulnerable to cross-site scripting attacks.

@exception {DeveloperError} viewer is required.

@example
const viewer = new Cesium.Viewer('cesiumContainer');
viewer.extend(Cesium.viewerPerformanceWatchdogMixin, {
    lowFrameRateMessage : 'Why is this going so <em>slowly</em>?'
});