DevTools: clear session id. follow-up patch after r183608

Source/core/inspector/InspectorTracingAgent.h

Index: Source/core/inspector/InspectorTracingAgent.h
diff --git a/Source/core/inspector/InspectorTracingAgent.h b/Source/core/inspector/InspectorTracingAgent.h
index 8469d18f093876282a12f3d7f98bb77005d7f3d5..5430fd892558df4096cc03d48351c4d440bfbb4f 100644
--- a/Source/core/inspector/InspectorTracingAgent.h
+++ b/Source/core/inspector/InspectorTracingAgent.h
@@ -30,6 +30,7 @@ public:
     // Base agent methods.
     virtual void restore() override;
     virtual void setFrontend(InspectorFrontend*) override;
+    virtual void clearFrontend() override;
 
     // Protocol method implementations.
     virtual void start(ErrorString*, const String& categoryFilter, const String&, const double*, PassRefPtrWillBeRawPtr<StartCallback>) override;
@@ -42,6 +43,7 @@ private:
     InspectorTracingAgent(InspectorClient*, InspectorWorkerAgent*);
 
     void emitMetadataEvents();
+    void resetSessionId();
     String sessionId();
 
     int m_layerTreeId;