[Offline Pages] Adds event logs for requests dropped due to number of start or complete attempts.

components/offline_pages/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_BACKGROUND_REQUEST_NOTIFIER_H_
 #define COMPONENTS_OFFLINE_PAGES_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.
   enum class BackgroundSavePageResult {
     SUCCESS,
     PRERENDER_FAILURE,
     PRERENDER_CANCELED,
     FOREGROUND_CANCELED,
     SAVE_FAILED,
     EXPIRED,
     RETRY_COUNT_EXCEEDED,
     START_COUNT_EXCEEDED,
     REMOVED,
   };
 
   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;
 };
 
 }  // namespace offline_pages
 
 #endif  // COMPONENTS_OFFLINE_PAGES_BACKGROUND_REQUEST_NOTIFIER_H_