[tracing] r39794 follow-up: Update the missing part of the test.

test/cctest/libplatform/test-tracing.cc

 // Copyright 2016 the V8 project 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 <limits>
 
 #include "include/libplatform/v8-tracing.h"
 #include "src/tracing/trace-event.h"
 #include "test/cctest/cctest.h"
 
 namespace v8 {
@@ skipping 354 matching lines @@
   default_platform->AddTraceStateObserver(&observer);
 
   CHECK_EQ(0, observer.enabled_count);
   CHECK_EQ(0, observer.disabled_count);
 
   tracing_controller.StartTracing(trace_config);
 
   CHECK_EQ(1, observer.enabled_count);
   CHECK_EQ(0, observer.disabled_count);
 
+  TraceStateObserverImpl observer2;
+  default_platform->AddTraceStateObserver(&observer2);
+
+  CHECK_EQ(1, observer2.enabled_count);
+  CHECK_EQ(0, observer2.disabled_count);
+
+  default_platform->RemoveTraceStateObserver(&observer2);
+
+  CHECK_EQ(1, observer2.enabled_count);
+  CHECK_EQ(0, observer2.disabled_count);
+
   tracing_controller.StopTracing();
 
   CHECK_EQ(1, observer.enabled_count);
   CHECK_EQ(1, observer.disabled_count);
+  CHECK_EQ(1, observer2.enabled_count);
+  CHECK_EQ(0, observer2.disabled_count);
 
   default_platform->RemoveTraceStateObserver(&observer);
 
   CHECK_EQ(1, observer.enabled_count);
   CHECK_EQ(1, observer.disabled_count);
 
   trace_config = new v8::platform::tracing::TraceConfig();
   tracing_controller.StartTracing(trace_config);
   tracing_controller.StopTracing();
 
   CHECK_EQ(1, observer.enabled_count);
   CHECK_EQ(1, observer.disabled_count);
 
   i::V8::SetPlatformForTesting(old_platform);
 }
 
 }  // namespace tracing
 }  // namespace platform
 }  // namespace v8