Add Mac memory pressure statistic reporting and consolidate platform code

base/memory/memory_pressure_monitor_unittest.cc

Index: base/memory/memory_pressure_monitor_unittest.cc
diff --git a/base/memory/memory_pressure_monitor_unittest.cc b/base/memory/memory_pressure_monitor_unittest.cc
new file mode 100644
index 0000000000000000000000000000000000000000..e9747418cc887383d822c9bf409861f8686df195
--- /dev/null
+++ b/base/memory/memory_pressure_monitor_unittest.cc
@@ -0,0 +1,33 @@
+// Copyright 2016 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/macros.h"
+#include "base/memory/memory_pressure_listener.h"
+#include "base/test/histogram_tester.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace base {
+
+TEST(MemoryPressureMonitorTest, RecordMemoryPressure) {
+  base::HistogramTester tester;
+  const char* kHistogram = "Memory.PressureLevel";
+
+  MemoryPressureMonitor::RecordMemoryPressure(
+      MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE, 3);
+  tester.ExpectTotalCount(kHistogram, 3);
+  tester.ExpectBucketCount(kHistogram, 0, 3);
+
+  MemoryPressureMonitor::RecordMemoryPressure(
+      MemoryPressureListener::MEMORY_PRESSURE_LEVEL_MODERATE, 2);
+  tester.ExpectTotalCount(kHistogram, 5);
+  tester.ExpectBucketCount(kHistogram, 1, 2);
+
+  MemoryPressureMonitor::RecordMemoryPressure(
+      MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL, 1);
+  tester.ExpectTotalCount(kHistogram, 6);
+  tester.ExpectBucketCount(kHistogram, 2, 1);
+}
+}  // namespace base