Update from https://crrev.com/328418

base/mac/memory_pressure_monitor_mac.cc

+// Copyright 2015 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.
+
+#include "base/mac/memory_pressure_monitor_mac.h"
+
+#include <dlfcn.h>
+#include <sys/sysctl.h>
+
+#include "base/mac/mac_util.h"
+
+namespace base {
+
+MemoryPressureListener::MemoryPressureLevel
+MemoryPressureMonitorMac::MemoryPressureLevelForMacMemoryPressure(
+    int mac_memory_pressure) {
+  switch (mac_memory_pressure) {
+    case DISPATCH_MEMORYPRESSURE_NORMAL:
+      return MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE;
+    case DISPATCH_MEMORYPRESSURE_WARN:
+      return MemoryPressureListener::MEMORY_PRESSURE_LEVEL_MODERATE;
+    case DISPATCH_MEMORYPRESSURE_CRITICAL:
+      return MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL;
+  }
+  return MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE;
+}
+
+void MemoryPressureMonitorMac::NotifyMemoryPressureChanged(
+    dispatch_source_s* event_source) {
+  int mac_memory_pressure = dispatch_source_get_data(event_source);
+  MemoryPressureListener::MemoryPressureLevel memory_pressure_level =
+      MemoryPressureLevelForMacMemoryPressure(mac_memory_pressure);
+  MemoryPressureListener::NotifyMemoryPressure(memory_pressure_level);
+}
+
+MemoryPressureMonitorMac::MemoryPressureMonitorMac()
+  : memory_level_event_source_(nullptr) {
+  // _dispatch_source_type_memorypressure is not available prior to 10.9.
+  dispatch_source_type_t dispatch_source_memorypressure =
+      static_cast<dispatch_source_type_t>
+          (dlsym(RTLD_NEXT, "_dispatch_source_type_memorypressure"));
+  if (dispatch_source_memorypressure) {
+    // The MemoryPressureListener doesn't want to know about transitions to
+    // MEMORY_PRESSURE_LEVEL_NONE so don't watch for
+    // DISPATCH_MEMORYPRESSURE_NORMAL notifications.
+    memory_level_event_source_.reset(
+        dispatch_source_create(dispatch_source_memorypressure, 0,
+                               DISPATCH_MEMORYPRESSURE_WARN |
+                                   DISPATCH_MEMORYPRESSURE_CRITICAL,
+                               dispatch_get_global_queue(
+                                   DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)));
+
+    dispatch_source_set_event_handler(memory_level_event_source_.get(), ^{
+        NotifyMemoryPressureChanged(memory_level_event_source_.get());
+    });
+    dispatch_retain(memory_level_event_source_.get());
+    dispatch_resume(memory_level_event_source_.get());
+  }
+}
+
+MemoryPressureMonitorMac::~MemoryPressureMonitorMac() {
+}
+
+MemoryPressureListener::MemoryPressureLevel
+MemoryPressureMonitorMac::GetCurrentPressureLevel() const {
+  if (base::mac::IsOSMountainLionOrEarlier()) {
+    return MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE;
+  }
+  int mac_memory_pressure;
+  size_t length = sizeof(int);
+  sysctlbyname("kern.memorystatus_vm_pressure_level", &mac_memory_pressure,
+               &length, nullptr, 0);
+  return MemoryPressureLevelForMacMemoryPressure(mac_memory_pressure);
+}
+
+}  // namespace base