Returns a CalendarButtonAutomationPeer for use by the Silverlight automation infrastructure.
This method creates a new CalendarButtonAutomationPeer instance if one has not been created for the CalendarButton; otherwise, it returns the CalendarButtonAutomationPeer previously created.
Classes that participate in the Silverlight automation infrastructure must implement this method to return a class-specific derived class of AutomationPeer that reports information for automation behavior.