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

Source/core/inspector/InspectorTracingAgent.cpp

 //
 // 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.
 //
 
 #include "config.h"
 
 #include "core/inspector/InspectorTracingAgent.h"
 
@@ skipping 20 matching lines @@
     , m_frontend(0)
     , m_workerAgent(workerAgent)
 {
 }
 
 void InspectorTracingAgent::restore()
 {
     emitMetadataEvents();
 }
 
-void InspectorTracingAgent::start(ErrorString*, const String& categoryFilter, const String&, const double*, PassRefPtrWillBeRawPtr<StartCallback> callback)
+void InspectorTracingAgent::start(ErrorString*, const String* categoryFilter, const String*, const double*, PassRefPtrWillBeRawPtr<StartCallback> callback)
 {
     ASSERT(m_state->getString(TracingAgentState::sessionId).isEmpty());
     m_state->setString(TracingAgentState::sessionId, IdentifiersFactory::createIdentifier());
-    m_client->enableTracing(categoryFilter);
+    m_client->enableTracing(categoryFilter ? *categoryFilter : String());
     emitMetadataEvents();
     callback->sendSuccess();
 }
 
 void InspectorTracingAgent::end(ErrorString* errorString, PassRefPtrWillBeRawPtr<EndCallback> callback)
 {
     m_client->disableTracing();
     resetSessionId();
     callback->sendSuccess();
 }
@@ skipping 27 matching lines @@
     resetSessionId();
 }
 
 void InspectorTracingAgent::resetSessionId()
 {
     m_state->remove(TracingAgentState::sessionId);
     m_workerAgent->setTracingSessionId(sessionId());
 }
 
 }