DevTools: respond with error when Debugger command is sent to disabled debugger agent

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 c2e2ed5ccc33f3cd8846c16bb7e502cb4afe741b..4e721e1d8ba3c8a13d6c45b09db7957f3b32d21e 100644
--- a/Source/core/inspector/InspectorDOMDebuggerAgent.h
+++ b/Source/core/inspector/InspectorDOMDebuggerAgent.h
@@ -105,6 +105,7 @@ public:
 
 private:
     InspectorDOMDebuggerAgent(InjectedScriptManager*, InspectorDOMAgent*, InspectorDebuggerAgent*);
+    bool checkEnabled(ErrorString*);
 
     void pauseOnNativeEventIfNeeded(PassRefPtr<JSONObject> eventData, bool synchronous);
     PassRefPtr<JSONObject> preparePauseOnNativeEventData(const String& eventName, const String* targetName);