Introduce base::MemoryPressureMonitor

This is a step towards unifying the various ways to "notify"
MemoryPressureListeners.

This also provides "GetCurrentPressureLevel" to a more general-purpose
place so we'll be able to use it on other platforms.
(Especially Windows, so we can use it in renderer_frame_manager).

See:
https://docs.google.com/a/chromium.org/document/d/1z3SR3JfKOwVaZxrsfHLOrPjo4-IVtnijaX86LKYPEQs/edit#
for more on the rationale and design, and
https://codereview.chromium.org/1045433002/
for what this might look like for ChromeOS's memory
monitor.

BUG=463546

Review URL: https://codereview.chromium.org/1048983006

Cr-Commit-Position: refs/heads/master@{#324086}

base/base.gypi

@@ -329,6 +329,8 @@
           'memory/manual_constructor.h',
           'memory/memory_pressure_listener.cc',
           'memory/memory_pressure_listener.h',
+          'memory/memory_pressure_monitor.cc',
+          'memory/memory_pressure_monitor.h',
           'memory/raw_scoped_refptr_mismatch_checker.h',
           'memory/ref_counted.cc',
           'memory/ref_counted.h',

base/memory/BUILD.gn

@@ -16,6 +16,8 @@ source_set("memory") {
     "manual_constructor.h",
     "memory_pressure_listener.cc",
     "memory_pressure_listener.h",
+    "memory_pressure_monitor.cc",
+    "memory_pressure_monitor.h",
     "raw_scoped_refptr_mismatch_checker.h",
     "ref_counted.cc",
     "ref_counted.h",

base/memory/memory_pressure_monitor.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/memory/memory_pressure_monitor.h"
+
+#include "base/logging.h"
+
+namespace base {
+namespace {
+
+MemoryPressureMonitor* g_monitor = nullptr;
+
+}  // namespace
+
+MemoryPressureMonitor::MemoryPressureMonitor() {
+  DCHECK(!g_monitor);
+  g_monitor = this;
+}
+
+MemoryPressureMonitor::~MemoryPressureMonitor() {
+  DCHECK(g_monitor);
+  g_monitor = nullptr;
+}
+
+// static
+MemoryPressureMonitor* MemoryPressureMonitor::Get() {
+  return g_monitor;
+}
+
+}  // namespace base

base/memory/memory_pressure_monitor.h

+// 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.
+
+#ifndef BASE_MEMORY_MEMORY_PRESSURE_MONITOR_H_
+#define BASE_MEMORY_MEMORY_PRESSURE_MONITOR_H_
+
+#include "base/base_export.h"
+#include "base/memory/memory_pressure_listener.h"
+
+namespace base {
+
+// Declares the interface for a MemoryPressureMonitor. There are multiple
+// OS specific implementations of this class. An instance of the memory
+// pressure observer is created at the process level, tracks memory usage, and
+// pushes memory state change notifications to the static function
+// base::MemoryPressureListener::NotifyMemoryPressure. This is turn notifies
+// all MemoryPressureListener instances via a callback.
+class BASE_EXPORT MemoryPressureMonitor {
+ public:
+  using MemoryPressureLevel = base::MemoryPressureListener::MemoryPressureLevel;
+
+  // Return the singleton MemoryPressureMonitor.
+  static MemoryPressureMonitor* Get();
+
+  // Returns the currently observed memory pressure.
+  virtual MemoryPressureLevel GetCurrentPressureLevel() const = 0;
+
+ protected:
+  MemoryPressureMonitor();
+  virtual ~MemoryPressureMonitor();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(MemoryPressureMonitor);
+};
+
+}  // namespace base
+
+#endif  // BASE_MEMORY_MEMORY_PRESSURE_MONITOR_H_