Enable overlapping, usage counted SetEnabled()/SetDisabled() for TraceLog

base/debug/trace_event_impl.h

Index: base/debug/trace_event_impl.h
diff --git a/base/debug/trace_event_impl.h b/base/debug/trace_event_impl.h
index 87cfa8cdfa2ead264fb8ac37c5d5e782660f51f5..fb9c5791021ed3b26ef34142f74264a45dc024a4 100644
--- a/base/debug/trace_event_impl.h
+++ b/base/debug/trace_event_impl.h
@@ -206,7 +206,7 @@ class BASE_EXPORT TraceLog {
   void SetDisabled();
   // Helper method to enable/disable tracing for all categories.
   void SetEnabled(bool enabled);
-  bool IsEnabled() { return enabled_; }
+  bool IsEnabled() { return !!enable_count_; }
 
 #if defined(OS_ANDROID)
   static void InitATrace();
@@ -367,7 +367,7 @@ class BASE_EXPORT TraceLog {
   // synchronization.
   // This lock protects TraceLog member accesses from arbitrary threads.
   Lock lock_;
-  bool enabled_;
+  int enable_count_;
   NotificationCallback notification_callback_;
   std::vector<TraceEvent> logged_events_;
   std::vector<std::string> included_categories_;