Create MemoryPressureStatsCollector.

components/memory_pressure/memory_pressure_stats_collector.h

Index: components/memory_pressure/memory_pressure_stats_collector.h
diff --git a/components/memory_pressure/memory_pressure_stats_collector.h b/components/memory_pressure/memory_pressure_stats_collector.h
new file mode 100644
index 0000000000000000000000000000000000000000..c777d5b9af60aa0ad1044201d2c4873ca72d2187
--- /dev/null
+++ b/components/memory_pressure/memory_pressure_stats_collector.h
@@ -0,0 +1,87 @@
+// 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 COMPONENTS_MEMORY_PRESSURE_MEMORY_PRESSURE_STATS_COLLECTOR_H_
+#define COMPONENTS_MEMORY_PRESSURE_MEMORY_PRESSURE_STATS_COLLECTOR_H_
+
+#include "base/time/tick_clock.h"
+#include "components/memory_pressure/memory_pressure_listener.h"
+
+namespace memory_pressure {
+
+// Enumeration of UMA memory pressure levels. This needs to be kept in sync with
+// histograms.xml and the memory pressure levels defined in

[grt (UTC plus 2), 2015/09/08 17:22:46]

these values aren't in sync with MemoryPressureLevel (-1, 0, and 2). please
either update the values or update the comment.

[chrisha, 2015/09/08 18:48:43]

Well, it needs to be kept in sync in the sense that there are corresponding
levels on both sides. The -1/0/2 is imposed by Android. UMA wants contiguous
buckets, hence 0/1/2.

I took a shot at improving the comment, but let me know if you have anything
particular in mind.

+// MemoryPressureListener. Included in the header so that
+// UMA_MEMORY_PRESSURE_LEVEL_COUNT is available.
+enum MemoryPressureLevelUMA {
+  UMA_MEMORY_PRESSURE_LEVEL_NONE = 0,
+  UMA_MEMORY_PRESSURE_LEVEL_MODERATE = 1,
+  UMA_MEMORY_PRESSURE_LEVEL_CRITICAL = 2,
+  // This must be the last value in the enum.
+  UMA_MEMORY_PRESSURE_LEVEL_COUNT,
+};
+
+// Enumeration of UMA pressure level changes. This needs to be kept in sync
+// with histograms.xml and the memory pressure levels defined in
+// MemoryPressureListener. Exposed for unittesting.
+enum MemoryPressureLevelChangeUMA {
+  UMA_MEMORY_PRESSURE_LEVEL_CHANGE_NONE_TO_MODERATE = 0,
+  UMA_MEMORY_PRESSURE_LEVEL_CHANGE_NONE_TO_CRITICAL = 1,
+  UMA_MEMORY_PRESSURE_LEVEL_CHANGE_MODERATE_TO_CRITICAL = 2,
+  UMA_MEMORY_PRESSURE_LEVEL_CHANGE_CRITICAL_TO_MODERATE = 3,
+  UMA_MEMORY_PRESSURE_LEVEL_CHANGE_CRITICAL_TO_NONE = 4,
+  UMA_MEMORY_PRESSURE_LEVEL_CHANGE_MODERATE_TO_NONE = 5,
+  // This must be the last value in the enum.
+  UMA_MEMORY_PRESSURE_LEVEL_CHANGE_COUNT
+};
+
+// Class that is responsible for collecting and eventually reporting memory

[grt (UTC plus 2), 2015/09/08 17:22:46]

nit: add the names of the histograms to the doc comment?

[chrisha, 2015/09/08 18:48:43]

Done.

+// pressure statistics.
+//
+// On platforms with a polling memory pressure implementation the
+// UpdateStatistics function will be invoked every time the pressure is polled.
+// On non-polling platforms (Mac, Android) it will be invoked on a periodic
+// timer, and at the moment of pressure level changes.
+class MemoryPressureStatsCollector {
+ public:
+  using MemoryPressureLevel = MemoryPressureListener::MemoryPressureLevel;
+
+  // The provided |tick_clock| must outlive this class.
+  MemoryPressureStatsCollector(base::TickClock* tick_clock);
+
+  // This is to be called periodically to ensure that up to date statistics
+  // have been reported.
+  void UpdateStatistics(MemoryPressureLevel current_pressure_level);
+
+ private:
+  friend class TestMemoryPressureStatsCollector;
+
+  // Helper functions for delivering collected statistics.
+  static void ReportCumulativeTime(MemoryPressureLevel pressure_level,
+                                   int seconds);
+  static void ReportLevelChange(MemoryPressureLevel old_pressure_level,
+                                MemoryPressureLevel new_pressure_level);
+
+  // The tick clock in use. This class is intended to be owned by a class with
+  // a tick clock, and ownership remains there. Also intended as a test seam.
+  base::TickClock* tick_clock_;
+
+  // Buckets of time that have been spent in different pressure levels, but
+  // not yet reported. At every call to UpdateStatistics these buckets will be
+  // drained of as many 'full' seconds of time as possible and reported via
+  // UMA. The remaining time (< 1000 ms) will be left in the bucket to be rolled
+  // into a later UMA report.
+  base::TimeDelta unreported_cumulative_time_[UMA_MEMORY_PRESSURE_LEVEL_COUNT];
+
+  // The last observed pressure level and the time at which it was observed, and
+  // the time when this pressure level started.
+  MemoryPressureLevel last_pressure_level_;
+  base::TimeTicks last_update_time_;
+
+  DISALLOW_COPY_AND_ASSIGN(MemoryPressureStatsCollector);
+};
+
+}  // namespace memory_pressure
+
+#endif  // COMPONENTS_MEMORY_PRESSURE_MEMORY_PRESSURE_STATS_COLLECTOR_H_