viewerPerformanceWatchdogMixin(viewer)
        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.
    | Name | Type | Default | Description | 
|---|---|---|---|
| viewer | Viewer | The viewer instance. | |
| options.lowFrameRateMessage | String | 'This application appears to be performing poorly on your system.  Please try using a different web browser or updating your video drivers.' | optional 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. | 
Throws:
- 
    DeveloperError : viewer is required.
Example:
var viewer = new Cesium.Viewer('cesiumContainer');
viewer.extend(Cesium.viewerPerformanceWatchdogMixin, {
    lowFrameRateMessage : 'Why is this going so <em>slowly</em>?'
});