Make memory pressure notifier configurable

base/memory/memory_pressure_monitor.h

Index: base/memory/memory_pressure_monitor.h
diff --git a/base/memory/memory_pressure_monitor.h b/base/memory/memory_pressure_monitor.h
index 6073bd3f44ca8f26b953a31967406c747e35bb58..a4adba4f3ad8d79913e1d2030568eead002d18b4 100644
--- a/base/memory/memory_pressure_monitor.h
+++ b/base/memory/memory_pressure_monitor.h
@@ -6,6 +6,7 @@
 #define BASE_MEMORY_MEMORY_PRESSURE_MONITOR_H_
 
 #include "base/base_export.h"
+#include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/memory_pressure_listener.h"
 
@@ -23,6 +24,7 @@ namespace base {
 class BASE_EXPORT MemoryPressureMonitor {
  public:
   using MemoryPressureLevel = base::MemoryPressureListener::MemoryPressureLevel;
+  using DispatchCallback = base::Callback<void(MemoryPressureLevel level)>;
 
   virtual ~MemoryPressureMonitor();
 
@@ -32,6 +34,10 @@ class BASE_EXPORT MemoryPressureMonitor {
   // Returns the currently observed memory pressure.
   virtual MemoryPressureLevel GetCurrentPressureLevel() const = 0;
 
+  // Sets a notification callback. The default callback invokes
+  // base::MemoryPressureListener::NotifyMemoryPressure.
+  virtual void SetDispatchCallback(const DispatchCallback& callback) = 0;
+
  protected:
   MemoryPressureMonitor();