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

Source/core/inspector/InspectorDOMDebuggerAgent.h

Index: Source/core/inspector/InspectorDOMDebuggerAgent.h
diff --git a/Source/core/inspector/InspectorDOMDebuggerAgent.h b/Source/core/inspector/InspectorDOMDebuggerAgent.h
index 4b4800e4cb209da2143529597b445ff0f698c7aa..d152e44d8a304246b5efc24fd5b5ae7a6dc10d94 100644
--- a/Source/core/inspector/InspectorDOMDebuggerAgent.h
+++ b/Source/core/inspector/InspectorDOMDebuggerAgent.h
@@ -93,7 +93,6 @@ public:
     void didProcessTask();
 
     virtual void clearFrontend();
-    virtual void discardAgent();
 
 private:
     InspectorDOMDebuggerAgent(InstrumentingAgents*, InspectorCompositeState*, InspectorDOMAgent*, InspectorDebuggerAgent*);
@@ -105,6 +104,8 @@ private:
     virtual void debuggerWasDisabled();
     virtual void stepInto();
     virtual void didPause();
+    virtual void willDestroyDebuggerAgent();
+
     void disable();
 
     void descriptionForDOMEvent(Node* target, int breakpointType, bool insertion, InspectorObject* description);