Improve the page download:

components/offline_pages/offline_page_model.h

 // 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.
 
 #ifndef COMPONENTS_OFFLINE_PAGES_OFFLINE_PAGE_MODEL_H_
 #define COMPONENTS_OFFLINE_PAGES_OFFLINE_PAGE_MODEL_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 63 matching lines @@
 
   // Returns true if saving an offline page may be attempted for |url|.
   static bool CanSaveURL(const GURL& url);
 
   OfflinePageModel();
   ~OfflinePageModel() override;
 
   virtual void AddObserver(Observer* observer) = 0;
   virtual void RemoveObserver(Observer* observer) = 0;
 
+  static const int64_t kInvalidOfflineId = 0;
+
   // Attempts to save a page addressed by |url| offline. Requires that the model
   // is loaded.  Generates a new offline id and returns
   // it. |proposed_offline_id| is used for the offline_id for the saved file if
-  // it is non-zero.  If it is zero, a new, random ID will be generated.
+  // it is non-zero.  If it is kInvalidOfflineId, a new, random ID will be
+  // generated.
   virtual void SavePage(const GURL& url,
                         const ClientId& client_id,
                         int64_t proposed_offline_id,
                         std::unique_ptr<OfflinePageArchiver> archiver,
                         const SavePageCallback& callback) = 0;
 
   // Marks that the offline page related to the passed |offline_id| has been
   // accessed. Its access info, including last access time and access count,
   // will be updated. Requires that the model is loaded.
   virtual void MarkPageAccessed(int64_t offline_id) = 0;
@@ skipping 65 matching lines @@
   // TODO(dougarnett): Remove this and its uses.
   virtual bool is_loaded() const = 0;
 
   // Returns the logger. Ownership is retained by the model.
   virtual OfflineEventLogger* GetLogger() = 0;
 };
 
 }  // namespace offline_pages
 
 #endif  // COMPONENTS_OFFLINE_PAGES_OFFLINE_PAGE_MODEL_H_