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

test/cctest/libplatform/test-tracing.cc

Index: test/cctest/libplatform/test-tracing.cc
diff --git a/test/cctest/libplatform/test-tracing.cc b/test/cctest/libplatform/test-tracing.cc
index 0ca51c308982c395722e8f71a360afaa5a00ae15..66fa0e05a524186ee1e7cddaa553a44cca0e09c6 100644
--- a/test/cctest/libplatform/test-tracing.cc
+++ b/test/cctest/libplatform/test-tracing.cc
@@ -372,10 +372,23 @@ TEST(TracingObservers) {
   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);