[ETW Export] Add polling for ETW keyword.

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 510113ae11fa6c9887a1ed050b235d2f4091bb31..dedebce937e671247e9a0e56a357272c872781b4 100644
--- a/base/trace_event/trace_log.h
+++ b/base/trace_event/trace_log.h
@@ -307,6 +307,13 @@ class BASE_EXPORT TraceLog : public MemoryDumpProvider {
   // may not handle the flush request in time causing lost of unflushed events.
   void SetCurrentThreadBlocksMessageLoop();
 
+#if defined(OS_WIN)
+  // This function is called by the ETW exporting module whenever the ETW
+  // keyword (flags) changes. This keyword indicates which categories should be
+  // exported, so whenever it changes, we adjust accordingly.
+  void UpdateETWCategoryGroupEnabledFlags();
+#endif
+
  private:
   typedef unsigned int InternalTraceOptions;