Make memory pressure notifier configurable

base/memory/memory_pressure_monitor_mac.h

Index: base/memory/memory_pressure_monitor_mac.h
diff --git a/base/memory/memory_pressure_monitor_mac.h b/base/memory/memory_pressure_monitor_mac.h
index b975b6ecec84ceb3dddef50fa1730fcf387ef2b4..62e0d83d0dd8d6e30de0347116d673e9049c4bdc 100644
--- a/base/memory/memory_pressure_monitor_mac.h
+++ b/base/memory/memory_pressure_monitor_mac.h
@@ -28,15 +28,21 @@ class BASE_EXPORT MemoryPressureMonitor : public base::MemoryPressureMonitor {
   // Returns the currently-observed memory pressure.
   MemoryPressureLevel GetCurrentPressureLevel() const override;
 
+  void SetDispatchCallback(DispatchCallback* callback) override;
+
  private:
   friend TestMemoryPressureMonitor;
 
   static MemoryPressureLevel
       MemoryPressureLevelForMacMemoryPressure(int mac_memory_pressure);
-  static void NotifyMemoryPressureChanged(dispatch_source_s* event_source);
+  static void NotifyMemoryPressureChanged(dispatch_source_s* event_source,
+                                          DispatchCallback dispatch_callback);
 
   ScopedDispatchObject<dispatch_source_t> memory_level_event_source_;
 
+  DispatchCallback dispatch_callback_ =
+      &MemoryPressureListener::NotifyMemoryPressure;
+
   DISALLOW_COPY_AND_ASSIGN(MemoryPressureMonitor);
 };