Ensure the 1993 NTP logs to UMA Most Visited tiles that use the fallback thumbnail instead of the p…

chrome/common/ntp_logging_events.h

 // Copyright 2013 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_COMMON_NTP_LOGGING_EVENTS_H_
 #define CHROME_COMMON_NTP_LOGGING_EVENTS_H_
 
 // The different types of events that are logged from the NTP.
 enum NTPLoggingEventType {
   // The user moused over an NTP tile or title.
   NTP_MOUSEOVER = 0,
 
   // The page attempted to load a thumbnail image.
   NTP_THUMBNAIL_ATTEMPT = 1,
 
-  // There was an error in loading a thumbnail image.
+  // There was an error in loading both the thumbnail image and the fallback
+  // (if it was provided), resulting in a grey tile.
   NTP_THUMBNAIL_ERROR = 2,
 
+  // The page attempted to load a thumbnail URL while a fallback thumbnail was
+  // provided.
+  NTP_FALLBACK_THUMBNAIL_REQUESTED = 3,
+
+  // The primary thumbnail image failed to load and caused us to use the
+  // secondary thumbnail as a fallback.
+  NTP_FALLBACK_THUMBNAIL_USED = 4,
+
   NTP_NUM_EVENT_TYPES
 };
 
 #endif  // CHROME_COMMON_NTP_LOGGING_EVENTS_H_