DevTools: add Tracing agent on back-end

Source/core/inspector/InspectorController.h

Index: Source/core/inspector/InspectorController.h
diff --git a/Source/core/inspector/InspectorController.h b/Source/core/inspector/InspectorController.h
index 0e0956b8ac561137e73e3e0a376a9b495efbf87d..92039c5009590ed90a37a5edc969ba882bbfdefc 100644
--- a/Source/core/inspector/InspectorController.h
+++ b/Source/core/inspector/InspectorController.h
@@ -55,6 +55,7 @@ class InspectorFrontendClient;
 class InspectorLayerTreeAgent;
 class InspectorPageAgent;
 class InspectorTimelineAgent;
+class InspectorTracingAgent;
 class InspectorOverlay;
 class InspectorState;
 class InstrumentingAgents;
@@ -152,6 +153,7 @@ private:
     InspectorPageAgent* m_pageAgent;
     InspectorTimelineAgent* m_timelineAgent;
     InspectorLayerTreeAgent* m_layerTreeAgent;
+    InspectorTracingAgent* m_tracingAgent;
 
     RefPtr<InspectorBackendDispatcher> m_inspectorBackendDispatcher;
     OwnPtr<InspectorFrontendClient> m_inspectorFrontendClient;