[MD History] Add UMA stats for switching views and the CBD button.

chrome/browser/resources/md_history/constants.js

 // 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.
 
 // Globals:
 /** @const */ var RESULTS_PER_PAGE = 150;
 
 /**
  * Amount of time between pageviews that we consider a 'break' in browsing,
  * measured in milliseconds.
  * @const
  */
 var BROWSING_GAP_TIME = 15 * 60 * 1000;
 
 /**
  * Maximum length of a history item title. Anything longer than this will be
  * cropped to fit within this limit. This value is large enough that it will not
  * be noticeable in a 960px wide history-item.
  * @const
  */
 var TITLE_MAX_LENGTH = 300;
 
 /**
+ * Histogram buckets for UMA tracking of which view is being shown to the user.
+ * Keep this in sync with the HistoryView enum in histograms.xml.

[Ilya Sherman, 2016/08/19 23:13:40]

Please also document that this enum should be treated as append-only.

[calamity, 2016/08/23 03:31:50]

Done.

+ * @enum {number}
+ */
+var HistoryViewHistogram = {
+  HISTORY: 0,
+  GROUPED_WEEK: 1,
+  GROUPED_MONTH: 2,
+  SYNCED_TABS: 3,
+  SIGNIN_PROMO: 4,
+  END: 5,  // Should always be last.
+};
+
+/**
  * @enum {number}
  */
 var HistoryRange = {
   ALL_TIME: 0,
   WEEK: 1,
   MONTH: 2
 };