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..762013e0b06901c8ae12429c0ad7abaaa49e0c5c
--- /dev/null
+++ b/base/memory/memory_pressure_monitor_unittest.cc
@@ -0,0 +1,33 @@
+// 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/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);
+  tester.ExpectTotalCount(kHistogram, 1);
+  tester.ExpectBucketCount(kHistogram, 0, 1);
+
+  MemoryPressureMonitor::RecordMemoryPressure(
+      MemoryPressureListener::MEMORY_PRESSURE_LEVEL_MODERATE);
+  tester.ExpectTotalCount(kHistogram, 2);
+  tester.ExpectBucketCount(kHistogram, 1, 1);
+
+  MemoryPressureMonitor::RecordMemoryPressure(
+      MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL);
+  tester.ExpectTotalCount(kHistogram, 3);
+  tester.ExpectBucketCount(kHistogram, 2, 1);
+}
+}  // namespace base