Make Computer a DescriptorByNameOwner

This will allow Computers to be used as AncestorInPath e.g. for
form validation.

core/src/main/java/hudson/model/Computer.java

@@ -147,7 +147,7 @@ import static javax.servlet.http.HttpServletResponse.*;
  * @author Kohsuke Kawaguchi
  */
 @ExportedBean
-public /*transient*/ abstract class Computer extends Actionable implements AccessControlled, ExecutorListener {
+public /*transient*/ abstract class Computer extends Actionable implements AccessControlled, ExecutorListener, DescriptorByNameOwner {
 
     private final CopyOnWriteArrayList<Executor> executors = new CopyOnWriteArrayList<Executor>();
     // TODO:
@@ -1624,6 +1624,11 @@ public /*transient*/ abstract class Computer extends Actionable implements Acces
         }
     }
 
+    @Override
+    public Descriptor getDescriptorByName(String className) {
+        return Jenkins.getInstance().getDescriptorByName(className);
+    }
+
     /**
      * A value class to provide a consistent snapshot view of the state of an executor to avoid race conditions
      * during rendering of the executors list.