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

Source/core/inspector/InspectorDebuggerAgent.h

 /*
  * Copyright (C) 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010-2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 116 matching lines @@
     void breakProgram(InspectorFrontend::Debugger::Reason::Enum breakReason, PassRefPtr<InspectorObject> data);
     virtual void scriptExecutionBlockedByCSP(const String& directiveText);
 
     class Listener {
     public:
         virtual ~Listener() { }
         virtual void debuggerWasEnabled() = 0;
         virtual void debuggerWasDisabled() = 0;
         virtual void stepInto() = 0;
         virtual void didPause() = 0;
+        virtual void willDestroyDebuggerAgent() = 0;
     };
     void setListener(Listener* listener) { m_listener = listener; }
 
     virtual ScriptDebugServer& scriptDebugServer() = 0;
 
     virtual void reportMemoryUsage(MemoryObjectInfo*) const;
 
     void setBreakpoint(const String& scriptId, int lineNumber, int columnNumber, BreakpointSource);
     void removeBreakpoint(const String& scriptId, int lineNumber, int columnNumber, BreakpointSource);
 
@@ skipping 50 matching lines @@
     InspectorFrontend::Debugger::Reason::Enum m_breakReason;
     RefPtr<InspectorObject> m_breakAuxData;
     bool m_javaScriptPauseScheduled;
     Listener* m_listener;
 };
 
 } // namespace WebCore
 
 
 #endif // !defined(InspectorDebuggerAgent_h)