Add TRACE_EVENT_IS_NEW_TRACE as a way to snapshot objects at start of recording

base/debug/trace_event_impl.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 
 #ifndef BASE_DEBUG_TRACE_EVENT_IMPL_H_
 #define BASE_DEBUG_TRACE_EVENT_IMPL_H_
 
 #include <stack>
 #include <string>
@@ skipping 299 matching lines @@
   Options trace_options() const { return trace_options_; }
 
   // Enables tracing. See CategoryFilter comments for details
   // on how to control what categories will be traced.
   void SetEnabled(const CategoryFilter& category_filter, Options options);
 
   // Disable tracing for all categories.
   void SetDisabled();
   bool IsEnabled() { return !!enable_count_; }
 
+  // The number of times we have begun recording traces. If tracing is off,
+  // returns -1. If tracing is on, then it returns the number of times we have
+  // recorded a trace. By watching for this number to increment, you can
+  // passively discover when a new trace has begun. This is then used to
+  // implement the TRACE_EVENT_IS_NEW_TRACE() primitive.
+  int GetNumTracesRecorded();
+
 #if defined(OS_ANDROID)
   void StartATrace();
   void StopATrace();
 #endif
 
   // Enabled state listeners give a callback when tracing is enabled or
   // disabled. This can be used to tie into other library's tracing systems
   // on-demand.
   class EnabledStateChangedObserver {
    public:
@@ skipping 187 matching lines @@
   static void ApplyATraceEnabledFlag(unsigned char* category_group_enabled);
 #endif
 
   TraceBuffer* GetTraceBuffer();
 
   // TODO(nduca): switch to per-thread trace buffers to reduce thread
   // synchronization.
   // This lock protects TraceLog member accesses from arbitrary threads.
   Lock lock_;
   int enable_count_;
+  int num_traces_recorded_;
   NotificationCallback notification_callback_;
   scoped_ptr<TraceBuffer> logged_events_;
   EventCallback event_callback_;
   bool dispatching_to_observer_list_;
   ObserverList<EnabledStateChangedObserver> enabled_state_observer_list_;
 
   base::hash_map<int, std::string> thread_names_;
   base::hash_map<int, std::stack<TimeTicks> > thread_event_start_times_;
   base::hash_map<std::string, int> thread_colors_;
 
@@ skipping 16 matching lines @@
 
   CategoryFilter category_filter_;
 
   DISALLOW_COPY_AND_ASSIGN(TraceLog);
 };
 
 }  // namespace debug
 }  // namespace base
 
 #endif  // BASE_DEBUG_TRACE_EVENT_IMPL_H_