Revert of Move tracing namespace from base::debug to base::trace_event.

base/trace_event/trace_event_system_stats_monitor.h

Index: base/trace_event/trace_event_system_stats_monitor.h
diff --git a/base/trace_event/trace_event_system_stats_monitor.h b/base/trace_event/trace_event_system_stats_monitor.h
index 051669a35f5ac1a5728c2929a13de10b0930ad31..08fbfea9331884bfe737356db523794bf7c31fff 100644
--- a/base/trace_event/trace_event_system_stats_monitor.h
+++ b/base/trace_event/trace_event_system_stats_monitor.h
@@ -17,7 +17,7 @@
 
 class SingleThreadTaskRunner;
 
-namespace trace_event {
+namespace debug {
 
 // Watches for chrome://tracing to be enabled or disabled. When tracing is
 // enabled, also enables system events profiling. This class is the preferred
@@ -35,7 +35,7 @@
 
   virtual ~TraceEventSystemStatsMonitor();
 
-  // base::trace_event::TraceLog::EnabledStateChangedObserver overrides:
+  // base::debug::TraceLog::EnabledStateChangedObserver overrides:
   void OnTraceLogEnabled() override;
   void OnTraceLogDisabled() override;
 
@@ -69,7 +69,7 @@
                                                   system_stats,
                                                   std::string* output);
 
-}  // namespace trace_event
+}  // namespace debug
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_TRACE_EVENT_SYSTEM_STATS_MONITOR_H_