Inspector: replace generic discardAgents logic with explicit call to discard a listener.

Source/core/inspector/InspectorCSSAgent.h

Index: Source/core/inspector/InspectorCSSAgent.h
diff --git a/Source/core/inspector/InspectorCSSAgent.h b/Source/core/inspector/InspectorCSSAgent.h
index 9fdd35cdd4003209b9a8823f35dde9faf5ed7e9f..528cb4235af60b9ca49549bb98450fe5a70deeaf 100644
--- a/Source/core/inspector/InspectorCSSAgent.h
+++ b/Source/core/inspector/InspectorCSSAgent.h
@@ -101,7 +101,6 @@ public:
     bool forcePseudoState(Element*, CSSSelector::PseudoType);
     virtual void setFrontend(InspectorFrontend*);
     virtual void clearFrontend();
-    virtual void discardAgent();
     virtual void restore();
     virtual void enable(ErrorString*);
     virtual void disable(ErrorString*);
@@ -180,6 +179,7 @@ private:
     virtual void didRemoveDocument(Document*);
     virtual void didRemoveDOMNode(Node*);
     virtual void didModifyDOMAttr(Element*);
+    virtual void discardListener();
 
     // InspectorCSSAgent::Listener implementation
     virtual void styleSheetChanged(InspectorStyleSheet*);