[ETW Export] Add polling for ETW keyword.

base/trace_event/trace_event_etw_export_win.h

Index: base/trace_event/trace_event_etw_export_win.h
diff --git a/base/trace_event/trace_event_etw_export_win.h b/base/trace_event/trace_event_etw_export_win.h
index a9b580c96090617d931f407f6ba1932a96e7d29a..3a6d08b7f1495ddf9ff5c140730de97cb403d1e2 100644
--- a/base/trace_event/trace_event_etw_export_win.h
+++ b/base/trace_event/trace_event_etw_export_win.h
@@ -10,6 +10,7 @@
 
 #include "base/base_export.h"
 #include "base/strings/string_piece.h"
+#include "base/timer/timer.h"
 #include "base/trace_event/trace_event_impl.h"
 
 // Fwd.
@@ -32,9 +33,9 @@ class BASE_EXPORT TraceEventETWExport {
   static void EnableETWExport();
   static void DisableETWExport();
 
-  static bool IsETWExportEnabled() {
-    return (GetInstance() && GetInstance()->etw_match_any_keyword_);
-  }
+  // Returns true if ETW is enabled. For now, this is true if the command line
+  // flag is specified.
+  static bool IsETWExportEnabled();
 
   // Exports an event to ETW. This is mainly used in
   // TraceLog::AddTraceEventWithThreadIdAndTimestamp to export internal events.
@@ -64,6 +65,11 @@ class BASE_EXPORT TraceEventETWExport {
   // Returns true if any category in the group is enabled.
   static bool IsCategoryGroupEnabled(const char* category_group_name);
 
+  // Starts the timer that keeps polls the keyword for changes.
+  static void StartKeywordUpdateTimer();
+
+  // Exposed for unittesting only, allows resurrecting our
+  // singleton instance post-AtExit processing
   void Resurrect();
 
  private:
@@ -72,11 +78,15 @@ class BASE_EXPORT TraceEventETWExport {
   TraceEventETWExport();
 
   // Updates the list of enabled categories by consulting the ETW keyword.
-  void UpdateEnabledCategories();
+  // Returns true if there was a change, false if unchanged.
+  bool UpdateEnabledCategories();
 
   // Returns true if the category is enabled.
   bool IsCategoryEnabled(const char* category_name) const;
 
+  // Called back by the timer to check for potential changes to the keyword.
+  static void KeywordUpdateTimerCallback();
+
   // True if ETW is enabled. Allows hiding the exporting behind a flag.
   bool etw_export_enabled_;
 
@@ -86,6 +96,9 @@ class BASE_EXPORT TraceEventETWExport {
   // Local copy of the ETW keyword.
   uint64 etw_match_any_keyword_;
 
+  // Repeating timer that is responsible for polling the keyword for changes.
+  base::RepeatingTimer<TraceEventETWExport> keyword_update_timer_;
+
   DISALLOW_COPY_AND_ASSIGN(TraceEventETWExport);
 };