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

Source/core/inspector/InspectorTracingAgent.cpp

Index: Source/core/inspector/InspectorTracingAgent.cpp
diff --git a/Source/core/inspector/InspectorTracingAgent.cpp b/Source/core/inspector/InspectorTracingAgent.cpp
index 6930a940d76c5664eabc0e7cd0d0133b56aa0f7a..bc18357efecfa1c2d7a1f0f9036a1287f9114455 100644
--- a/Source/core/inspector/InspectorTracingAgent.cpp
+++ b/Source/core/inspector/InspectorTracingAgent.cpp
@@ -38,11 +38,11 @@ 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();
 }