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

Source/core/inspector/InspectorDebuggerAgent.h

Index: Source/core/inspector/InspectorDebuggerAgent.h
diff --git a/Source/core/inspector/InspectorDebuggerAgent.h b/Source/core/inspector/InspectorDebuggerAgent.h
index 42d85b5d5898ffd8c2c4e858f9b34db0d845dcc7..c8891cdce9b2934d406bc102f293186a7412437f 100644
--- a/Source/core/inspector/InspectorDebuggerAgent.h
+++ b/Source/core/inspector/InspectorDebuggerAgent.h
@@ -134,6 +134,7 @@ public:
         virtual void debuggerWasDisabled() = 0;
         virtual void stepInto() = 0;
         virtual void didPause() = 0;
+        virtual void willDestroyDebuggerAgent() = 0;
     };
     void setListener(Listener* listener) { m_listener = listener; }