Add UMA metrics for desktop memory pressure notifications.

content/child/memory/child_memory_message_filter.h

Index: content/child/memory/child_memory_message_filter.h
diff --git a/content/child/memory/child_memory_message_filter.h b/content/child/memory/child_memory_message_filter.h
index de9c3b5f6742ca01f02e7c6b59a75cd6cb12b6b7..77562024d5cc6b0a5bf910ece99b972c50b7e4f0 100644
--- a/content/child/memory/child_memory_message_filter.h
+++ b/content/child/memory/child_memory_message_filter.h
@@ -7,6 +7,7 @@
 
 #include "base/macros.h"
 #include "base/memory/memory_pressure_listener.h"
+#include "base/timer/timer.h"
 #include "ipc/message_filter.h"
 
 namespace content {
@@ -30,6 +31,10 @@ class ChildMemoryMessageFilter : public IPC::MessageFilter {
   void OnPressureNotification(
       base::MemoryPressureListener::MemoryPressureLevel level);
 
+#if defined(OS_MACOSX)
+  base::Timer pressure_timer_;
+#endif
+
   DISALLOW_COPY_AND_ASSIGN(ChildMemoryMessageFilter);
 };