Add a trace event to MemoryPressureListener::NotifyMemoryPressure.

Add a trace event to MemoryPressureListener::NotifyMemoryPressure to enable
tracking of whether any jank is caused by dealing with memory pressure
notifications.

BUG=397026

Review URL: https://codereview.chromium.org/413233002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285449 0039d316-1c4b-4281-b951-d872f2087c98

base/memory/memory_pressure_listener.cc

@@ -4,6 +4,7 @@
 
 #include "base/memory/memory_pressure_listener.h"
 
+#include "base/debug/trace_event.h"
 #include "base/lazy_instance.h"
 #include "base/observer_list_threadsafe.h"
 
@@ -50,6 +51,8 @@ void MemoryPressureListener::Notify(MemoryPressureLevel memory_pressure_level) {
 // static
 void MemoryPressureListener::NotifyMemoryPressure(
     MemoryPressureLevel memory_pressure_level) {
+  TRACE_EVENT1("memory", "MemoryPressureListener::NotifyMemoryPressure",
+      "level", memory_pressure_level);
   g_observers.Get().Notify(&MemoryPressureListener::Notify,
                            memory_pressure_level);
 }