[DevTools] Make all parameters of Tracing.start optional.

Source/core/inspector/InspectorTracingAgent.h

 /*
  * Copyright 2014 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef InspectorTracingAgent_h
 #define InspectorTracingAgent_h
 
 #include "core/InspectorFrontend.h"
@@ skipping 15 matching lines @@
     {
         return adoptPtrWillBeNoop(new InspectorTracingAgent(client, workerAgent));
     }
 
     // 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;
+    virtual void start(ErrorString*, const String* categoryFilter, const String*, const double*, PassRefPtrWillBeRawPtr<StartCallback>) override;
     virtual void end(ErrorString*, PassRefPtrWillBeRawPtr<EndCallback>);
 
     // Methods for other agents to use.
     void setLayerTreeId(int);
 
 private:
     InspectorTracingAgent(InspectorClient*, InspectorWorkerAgent*);
 
     void emitMetadataEvents();
     void resetSessionId();
     String sessionId();
 
     int m_layerTreeId;
     InspectorClient* m_client;
     InspectorFrontend::Tracing* m_frontend;
     InspectorWorkerAgent* m_workerAgent;
 };
 
 }
 
 #endif // InspectorTracingAgent_h