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

Source/core/inspector/InspectorDebuggerAgent.cpp

 /*
  * Copyright (C) 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 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 162 matching lines @@
 }
 
 void InspectorDebuggerAgent::init()
 {
     m_promiseTracker = PromiseTracker::create(this, scriptDebugServer().isolate());
     // FIXME: make breakReason optional so that there was no need to init it with "other".
     clearBreakDetails();
     m_state->setLong(DebuggerAgentState::pauseOnExceptionsState, ScriptDebugServer::DontPauseOnExceptions);
 }
 
+bool InspectorDebuggerAgent::checkEnabled(ErrorString* errorString)
+{
+    if (enabled())
+        return true;
+    *errorString = "Debugger agent is not enabled";
+    return false;
+}
+
 void InspectorDebuggerAgent::enable()
 {
     m_instrumentingAgents->setInspectorDebuggerAgent(this);
 
     startListeningScriptDebugServer();
     // FIXME(WK44513): breakpoints activated flag should be synchronized between all front-ends
     scriptDebugServer().setBreakpointsActivated(true);
 
     m_state->setBoolean(DebuggerAgentState::debuggerEnabled, true);
     if (m_listener)
@@ skipping 1435 matching lines @@
     visitor->trace(m_v8AsyncCallTracker);
     visitor->trace(m_promiseTracker);
     visitor->trace(m_asyncOperations);
     visitor->trace(m_currentAsyncCallChain);
     visitor->trace(m_asyncCallTrackingListeners);
 #endif
     InspectorBaseAgent::trace(visitor);
 }
 
 } // namespace blink