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

Source/core/inspector/InspectorDOMDebuggerAgent.cpp

Index: Source/core/inspector/InspectorDOMDebuggerAgent.cpp
diff --git a/Source/core/inspector/InspectorDOMDebuggerAgent.cpp b/Source/core/inspector/InspectorDOMDebuggerAgent.cpp
index 3a346da3fd32005eb339a367ad8ca70cfa54495f..6273b7116fca3e3a1ec83f2533a61d1e00aca2c7 100644
--- a/Source/core/inspector/InspectorDOMDebuggerAgent.cpp
+++ b/Source/core/inspector/InspectorDOMDebuggerAgent.cpp
@@ -137,6 +137,19 @@ void InspectorDOMDebuggerAgent::domAgentWasDisabled()
     disable(nullptr);
 }
 
+bool InspectorDOMDebuggerAgent::checkEnabled(ErrorString* errorString)
+{
+    if (!m_domAgent->enabled()) {
+        *errorString = "DOM domain required by DOMDebugger is not enabled";
+        return false;
+    }
+    if (!m_debuggerAgent->enabled()) {
+        *errorString = "Debugger domain required by DOMDebugger is not enabled";
+        return false;
+    }
+    return true;
+}
+
 void InspectorDOMDebuggerAgent::disable(ErrorString*)
 {
     m_instrumentingAgents->setInspectorDOMDebuggerAgent(nullptr);