[ETW Export] Add polling for ETW keyword.

content/browser/browser_main_loop.cc

Index: content/browser/browser_main_loop.cc
diff --git a/content/browser/browser_main_loop.cc b/content/browser/browser_main_loop.cc
index 1206fd6954df1d5dbf9712eddb9c0562f66a8b86..59a0e309ed5254451f9f831c65f508f54627cf01 100644
--- a/content/browser/browser_main_loop.cc
+++ b/content/browser/browser_main_loop.cc
@@ -26,6 +26,7 @@
 #include "base/timer/hi_res_timer_manager.h"
 #include "base/trace_event/memory_dump_manager.h"
 #include "base/trace_event/trace_event.h"
+#include "base/trace_event/trace_event_etw_export_win.h"
 #include "content/browser/browser_thread_impl.h"
 #include "content/browser/device_sensors/device_inertial_sensor_service.h"
 #include "content/browser/dom_storage/dom_storage_area.h"
@@ -697,6 +698,12 @@ int BrowserMainLoop::PreCreateThreads() {
     RenderProcessHost::SetRunRendererInProcess(true);
 #endif
 
+#if defined(OS_WIN)
+  // Starts a timer to poll the ETW keyword (flags), that checks if a consumer
+  // is listening and if the categories have changed.
+  base::trace_event::TraceEventETWExport::StartKeywordUpdateTimer();

[brucedawson, 2015/06/26 22:24:24]

Does this get called in just the browser or in all processes?

If in just the browser then the other processes will never emit events.
If in all processes then we can end up with arbitrarily many wakeups per second.

Ideal implementation would do the timer-based checking in just the browser
process and then send an IPC message to the other processes when there is a
change. I'm not sure if that is overkill or ideal.

+#endif
+
   return result_code_;
 }