Add UMA metrics to the site engagement service.

chrome/browser/engagement/site_engagement_metrics.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 CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_METRICS_H_
+#define CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_METRICS_H_
+
+#include <map>
+
+#include "url/gurl.h"
+
+// Helper class managing the UMA histograms for the Site Engagement Service.
+class SiteEngagementMetrics {
+ public:
+  // This is used to back a UMA histogram, so it should be treated as
+  // append-only.
+  enum EngagementType {
+    ENGAGEMENT_NAVIGATION,
+    ENGAGEMENT_KEYPRESS,
+    ENGAGEMENT_MOUSE,
+  };
+
+  static void RecordTotalSiteEngagement(double total_engagement);
+
+  static void RecordTotalOriginsEngaged(int total_origins);

[benwells, 2015/09/30 00:10:54]

Nit: I don' think you need a blank line between every function. It's good to
group related things, you could easily say these are all related so should all
be grouped. Or maybe the Max ones are a group.

[dominickn, 2015/10/01 01:12:02]

Done.

+
+  static void RecordEngagementScore(std::map<GURL, double> score_map);

[benwells, 2015/09/30 00:10:54]

Nit: this should be RecordEngagementScores (i.e. with an S at the end).

[dominickn, 2015/10/01 01:12:02]

Done.

+
+  static void RecordOriginsWithMaxEngagement(int total_origins);
+
+  static void RecordOriginsWithMaxDailyEngagement(int total_origins);
+
+  static void RecordEngagement(EngagementType type);
+
+  static const char kTotalEngagementHistogram[];

[benwells, 2015/09/30 00:10:54]

Nit: normally you'd put these first (and that would match the .cc file order).

[dominickn, 2015/10/01 01:12:02]

Done.

+  static const char kTotalOriginsHistogram[];
+  static const char kEngagementScoreHistogram[];
+  static const char kOriginsWithMaxEngagementHistogram[];
+  static const char kOriginsWithMaxDailyEngagementHistogram[];
+  static const char kEngagementTypeHistogram[];
+

[benwells, 2015/09/30 00:10:54]

Nit: no blank line.

[dominickn, 2015/10/01 01:12:02]

Done.

+};
+
+#endif  // CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_METRICS_H_