Update from https://crrev.com/312600

base/chromeos/memory_pressure_observer_chromeos.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_CHROMEOS_MEMORY_PRESSURE_OBSERVER_CHROMEOS_H_
 #define BASE_CHROMEOS_MEMORY_PRESSURE_OBSERVER_CHROMEOS_H_
 
 #include "base/base_export.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/memory_pressure_listener.h"
 #include "base/memory/weak_ptr.h"
 #include "base/timer/timer.h"
 
 namespace base {
 
 class TestMemoryPressureObserver;
 
 ////////////////////////////////////////////////////////////////////////////////
 // MemoryPressureObserverChromeOS
 //
 // A class to handle the observation of our free memory. It notifies the
 // MemoryPressureListener of memory fill level changes, so that it can take
 // action to reduce memory resources accordingly.
 //
 class BASE_EXPORT MemoryPressureObserverChromeOS {
  public:
   using GetUsedMemoryInPercentCallback = int (*)();
 
-  MemoryPressureObserverChromeOS();
+  // There are two memory pressure events:
+  // MODERATE - which will mainly release caches.
+  // CRITICAL - which will discard tabs.
+  // The |MemoryPressureThresholds| enum selects the strategy of firing these
+  // events: A conservative strategy will keep as much content in memory as
+  // possible (causing the system to swap to zram) and an aggressive strategy
+  // will release memory earlier to avoid swapping.
+  enum MemoryPressureThresholds {
+    // Use the system default.
+    THRESHOLD_DEFAULT = 0,
+    // Try to keep as much content in memory as possible.
+    THRESHOLD_CONSERVATIVE = 1,
+    // Discard caches earlier, allowing to keep more tabs in memory.
+    THRESHOLD_AGGRESSIVE_CACHE_DISCARD = 2,
+    // Discard tabs earlier, allowing the system to get faster.
+    THRESHOLD_AGGRESSIVE_TAB_DISCARD = 3,
+    // Discard caches and tabs earlier to allow the system to be faster.
+    THRESHOLD_AGGRESSIVE = 4
+  };
+
+  explicit MemoryPressureObserverChromeOS(MemoryPressureThresholds thresholds);
   virtual ~MemoryPressureObserverChromeOS();
 
   // Redo the memory pressure calculation soon and call again if a critical
   // memory pressure prevails. Note that this call will trigger an asynchronous
   // action which gives the system time to release memory back into the pool.
   void ScheduleEarlyCheck();
 
   // Get the current memory pressure level.
   base::MemoryPressureListener::MemoryPressureLevel GetCurrentPressureLevel() {
     return current_memory_pressure_level_;
@@ skipping 23 matching lines @@
       current_memory_pressure_level_;
 
   // A periodic timer to check for resource pressure changes. This will get
   // replaced by a kernel triggered event system (see crbug.com/381196).
   base::RepeatingTimer<MemoryPressureObserverChromeOS> timer_;
 
   // To slow down the amount of moderate pressure event calls, this counter
   // gets used to count the number of events since the last event occured.
   int moderate_pressure_repeat_count_;
 
+  // The thresholds for moderate and critical pressure.
+  const int moderate_pressure_threshold_percent_;
+  const int critical_pressure_threshold_percent_;
+
   base::WeakPtrFactory<MemoryPressureObserverChromeOS> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MemoryPressureObserverChromeOS);
 };
 
 }  // namespace base
 
 #endif  // BASE_CHROMEOS_MEMORY_PRESSURE_OBSERVER_CHROMEOS_H_