Add OnBeforeTraceLogDisabled notification to EnabledStateObserver

base/trace_event/trace_log.h

Index: base/trace_event/trace_log.h
diff --git a/base/trace_event/trace_log.h b/base/trace_event/trace_log.h
index 540d7d0bd3e0e06aa910b65cb2850ef08143cc62..9f4aaff3c1ea8038d60cfcdcde6870ccbdcb8dd8 100644
--- a/base/trace_event/trace_log.h
+++ b/base/trace_event/trace_log.h
@@ -113,6 +113,11 @@ class BASE_EXPORT TraceLog : public MemoryDumpProvider {
     // |lock_|. TraceLog::IsEnabled() is true at this point.
     virtual void OnTraceLogEnabled() = 0;
 
+    // Called just before the tracing system disables, outside of the |lock_|.
+    // TraceLog::IsEnabled() is true at this point. It is still possible to
+    // emit trace events from the observer.
+    virtual void OnBeforeTraceLogDisabled() {}
+
     // Called just after the tracing system disables, outside of the |lock_|.
     // TraceLog::IsEnabled() is false at this point.
     virtual void OnTraceLogDisabled() = 0;
@@ -480,6 +485,10 @@ class BASE_EXPORT TraceLog : public MemoryDumpProvider {
   // This lock protects TraceLog member accesses (except for members protected
   // by thread_info_lock_) from arbitrary threads.
   mutable Lock lock_;
+  // This lock protects TraceLog::SetDisable from being entered by multiple
+  // threads. The generic |lock_| could be unlocked during iterating over
+  // EnabledStateObserver's.
+  mutable Lock disabling_lock_;
   // This lock protects accesses to thread_names_, thread_event_start_times_
   // and thread_colors_.
   Lock thread_info_lock_;