[8u40] Post-commit review: RT-38640: Improve handling of Control.getUserAgentStylesheet()

[David Grieve]

A change was made to the Control API which removed the protected method 
getUserAgentStylesheet(). This method was  moved to Region and was made 
public:

javafx.scene.control.Control
-    protected String getUserAgentStylesheet()

javafx.scene.layout.Region
+    public String getUserAgentStylesheet()

Please note that while this is a binary-compatible change, it is not a 
source compatible change. Code which has overridden the protected 
getUserAgentStylesheet() method from Control will continue to run, but 
will no longer compile in 8u40. The access on the overridden method must 
be changed to public.

The benefit of this change is that CSS can be easily packaged with 
custom controls.