Add "UNKNOWN_TILE_TYPE" for metrics

components/ntp_tiles/metrics.h

 // 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.
 
 #ifndef COMPONENTS_NTP_TILES_METRICS_H_
 #define COMPONENTS_NTP_TILES_METRICS_H_
 
 #include <utility>
 #include <vector>
 
@@ skipping 16 matching lines @@
   ICON_REAL,
   // The item displays a color derived from the site's favicon or touch icon.
   ICON_COLOR,
   // The item displays a default gray box in place of an icon.
   ICON_DEFAULT,
   // The number of different tile types that get recorded. Entries below this
   // are not recorded in UMA.
   NUM_RECORDED_TILE_TYPES,
   // The item displays a thumbnail of the page. Used on desktop.
   THUMBNAIL,
+  // The tile type has not been determined yet. Used on iOS, until we can detect
+  // when all tiles have loaded.
+  UNKNOWN_TILE_TYPE,
 };
 
 // Records an NTP impression, after all tiles have loaded.
 // Includes the visual types (see above) of all visible tiles.
 void RecordPageImpression(
     const std::vector<std::pair<NTPTileSource, MostVisitedTileType>>& tiles);
 
 // Records a click on a tile.
 void RecordTileClick(int index,
                      NTPTileSource source,
                      MostVisitedTileType tile_type);
 
 }  // namespace metrics
 }  // namespace ntp_tiles
 
 #endif