NTP: Fix TileType metrics on desktop

chrome/browser/resources/local_ntp/most_visited_single.js

 /* 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. */
 
  // Single iframe for NTP tiles.
 (function() {
 'use strict';
 
 
 /**
@@ skipping 33 matching lines @@
  * The different (visual) types that an NTP tile can have.
  * Note: Keep in sync with components/ntp_tiles/tile_visual_type.h
  * @enum {number}
  * @const
  */
 var TileVisualType = {
   NONE: 0,
   ICON_REAL: 1,
   ICON_COLOR: 2,
   ICON_DEFAULT: 3,
-  THUMBNAIL: 4,
-  THUMBNAIL_FAILED: 5,
+  THUMBNAIL: 7,
+  THUMBNAIL_FAILED: 8,

[Ilya Sherman, 2017/04/21 19:58:38]

I'm confused about why there is a gap in this enum.  It sounds like there's a
Java-side enum as well, and the two need to be kept in sync?  That sounds really
fragile.  If my understanding is correct, please move all of the enumerated
constants into C++, and expose them to Java.  (I know that I've seen this done
elsewhere in the codebase, though I don't have a reference handy,
unfortunately.)

Also, if my understanding is correct, then the histograms recording these
metrics are going to have conflated data for some buckets.  The best practice in
this case is to rename the histograms, so that the active histograms have
internally consistent semantics.

 };
 
 
 /**
  * Total number of tiles to show at any time. If the host page doesn't send
  * enough tiles, we fill them blank.
  * @const {number}
  */
 var NUMBER_OF_TILES = 8;
 
@@ skipping 481 matching lines @@
     var html = document.querySelector('html');
     html.dir = 'rtl';
   }
 
   window.addEventListener('message', handlePostMessage);
 };
 
 
 window.addEventListener('DOMContentLoaded', init);
 })();