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

The test now checks the case when an observer is added after tracing is started.

BUG=chromium:406277

Review-Url: https://codereview.chromium.org/2376953002
Cr-Commit-Position: refs/heads/master@{#39874}

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);