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

Source/core/inspector/InspectorDebuggerAgent.cpp

Index: Source/core/inspector/InspectorDebuggerAgent.cpp
diff --git a/Source/core/inspector/InspectorDebuggerAgent.cpp b/Source/core/inspector/InspectorDebuggerAgent.cpp
index eaead68db7c6ff416a0fc0405b97d84c8e6a756e..82becf4f634e091fc098f1898f08f964203f6b85 100644
--- a/Source/core/inspector/InspectorDebuggerAgent.cpp
+++ b/Source/core/inspector/InspectorDebuggerAgent.cpp
@@ -180,6 +180,14 @@ void InspectorDebuggerAgent::init()
     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);