[Offline Pages] Add UMA to record final offline request status and loading failed error code.

components/offline_pages/core/background/request_notifier.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_OFFLINE_PAGES_CORE_BACKGROUND_REQUEST_NOTIFIER_H_
 #define COMPONENTS_OFFLINE_PAGES_CORE_BACKGROUND_REQUEST_NOTIFIER_H_
 
 namespace offline_pages {
 
 class SavePageRequest;
 
 class RequestNotifier {
  public:
   // Status to return for failed notifications.
   // NOTE: for any changes to the enum, please also update related switch code
   // in RequestCoordinatorEventLogger.
   // GENERATED_JAVA_ENUM_PACKAGE:org.chromium.components.offlinepages
+  // WARNING: You must update histograms.xml to match any changes made to
+  // this enum (ie, histogram enum for OfflinePagesBackgroundSavePageResult).
   enum class BackgroundSavePageResult {
-    SUCCESS,
-    LOADING_FAILURE,
-    LOADING_CANCELED,
-    FOREGROUND_CANCELED,
-    SAVE_FAILED,
-    EXPIRED,
-    RETRY_COUNT_EXCEEDED,
-    START_COUNT_EXCEEDED,
-    REMOVED,
+    SUCCESS = 0,
+    LOADING_FAILURE = 1,
+    LOADING_CANCELED = 2,
+    FOREGROUND_CANCELED = 3,
+    SAVE_FAILED = 4,
+    EXPIRED = 5,
+    RETRY_COUNT_EXCEEDED = 6,
+    START_COUNT_EXCEEDED = 7,
+    REMOVED = 8,
+    // NOTE: insert new values above this line and update histogram enum too.
+    STATUS_COUNT
   };
 
   virtual ~RequestNotifier() = default;
 
   // Notifies observers that |request| has been added.
   virtual void NotifyAdded(const SavePageRequest& request) = 0;
 
   // Notifies observers that |request| has been completed with |status|.
   virtual void NotifyCompleted(const SavePageRequest& request,
                                BackgroundSavePageResult status) = 0;
 
   // Notifies observers that |request| has been changed.
   virtual void NotifyChanged(const SavePageRequest& request) = 0;
 
   // Notifies observers of progress on |request|, which received
   // |received_bytes| at that point.
   virtual void NotifyNetworkProgress(const SavePageRequest& request,
                                      int64_t received_bytes) = 0;
 };
 
 }  // namespace offline_pages
 
 #endif  // COMPONENTS_OFFLINE_PAGES_CORE_BACKGROUND_REQUEST_NOTIFIER_H_