netlog: Don't enable recording if the "netlog" tracing category is disabled.

net/log/trace_net_log_observer_unittest.cc

Index: net/log/trace_net_log_observer_unittest.cc
diff --git a/net/log/trace_net_log_observer_unittest.cc b/net/log/trace_net_log_observer_unittest.cc
index 2cd4f01a867d41f072a501b9604576fdc5013c28..68e3131af6e0c29d986c3c4d5e9d7b78f17e47e4 100644
--- a/net/log/trace_net_log_observer_unittest.cc
+++ b/net/log/trace_net_log_observer_unittest.cc
@@ -347,6 +347,27 @@ TEST_F(TraceNetLogObserverTest,
   EXPECT_EQ(0u, trace_events()->GetSize());
 }
 
+TEST_F(TraceNetLogObserverTest, DisabledCategoryDoesntEnableNetLog) {

[xunjieli, 2016/12/05 14:55:01]

Thanks for working on this. The change looks good. 
We should test whether NetLog::IsCapturing() is set accordingly. If
IsCapturing(), it means that we have at least one observer. However, since the
TraceNetLogObserverTest fixture always has a TestNetLog so it will always have
an observer.

Could you use TEST instead of TEST_P (to avoid instantiating a TestNetLog) and
do something along the following lines?

TEST(TraceNetLogEnableTest, DisabledCategory) {
  TraceNetLogObserver observer;
  NetLog net_log;  
  observer.WatchForTraceStart(&net_log);

  EXPECT_FALSE(net_log.IsCapturing());
  std::string disabled_netlog_category =
      std::string("-") + kNetLogTracingCategory;
  TraceLog::GetInstance()->SetEnabled(
      base::trace_event::TraceConfig(disabled_netlog_category, ""),
      TraceLog::RECORDING_MODE);
  net_log.AddGlobalEntry(NetLogEventType::REQUEST_ALIVE);

  EXPECT_FALSE(net_log.IsCapturing());

  observer.StopWatchForTraceStart();

  EXPECT_FALSE(net_log.IsCapturing());
}


And another test where netlog category is enabled and we have
net_log.IsCapturing() equals to true before StopWatchForTraceStart().

+  set_trace_net_log_observer(new TraceNetLogObserver());
+  trace_net_log_observer()->WatchForTraceStart(net_log());
+
+  std::string disabled_netlog_category =
+      std::string("-") + kNetLogTracingCategory;
+  TraceLog::GetInstance()->SetEnabled(
+      base::trace_event::TraceConfig(disabled_netlog_category, ""),
+      TraceLog::RECORDING_MODE);
+
+  net_log()->AddGlobalEntry(NetLogEventType::REQUEST_ALIVE);
+
+  trace_net_log_observer()->StopWatchForTraceStart();
+
+  EndTraceAndFlush();
+  TestNetLogEntry::List entries;
+  net_log()->GetEntries(&entries);
+  EXPECT_EQ(1u, entries.size());
+  EXPECT_EQ(0u, trace_events()->GetSize());
+}
+
 TEST_F(TraceNetLogObserverTest, EventsWithAndWithoutParameters) {
   trace_net_log_observer()->WatchForTraceStart(net_log());
   EnableTraceLog();