Failed offline snapshots won't erase a successful one from the same page load.

chrome/browser/android/offline_pages/recent_tab_helper.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 CHROME_BROWSER_ANDROID_OFFLINE_PAGES_RECENT_TAB_HELPER_H_
 #define CHROME_BROWSER_ANDROID_OFFLINE_PAGES_RECENT_TAB_HELPER_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 80 matching lines @@
   };
 
   explicit RecentTabHelper(content::WebContents* web_contents);
   friend class content::WebContentsUserData<RecentTabHelper>;
 
   void EnsureInitialized();
   void ContinueSnapshotWithIdsToPurge(const std::vector<int64_t>& page_ids);
   void ContinueSnapshotAfterPurge(OfflinePageModel::DeletePageResult result);
   void SavePageCallback(OfflinePageModel::SavePageResult result,
                         int64_t offline_id);
+  void PreviousSaveWasPurged(OfflinePageModel::DeletePageResult result);
   void ReportSnapshotCompleted();
   void ReportDownloadStatusToRequestCoordinator();
   bool IsSamePage() const;
   ClientId GetRecentPagesClientId() const;
 
   // Page model is a service, no ownership. Can be null - for example, in
   // case when tab is in incognito profile.
   OfflinePageModel* page_model_;
 
   // If false, never make snapshots off the attached WebContents.
   // Not page-specific.
   bool snapshots_enabled_;
 
   // Becomes true during navigation if the page is ready for snapshot as
   // indicated by at least one callback from SnapshotController.
   bool is_page_ready_for_snapshot_;
 
   // Info for the offline page to capture. Null if the tab is not capturing
   // current page.
   std::unique_ptr<DownloadPageInfo> download_info_;
 
+  // Stores the request/offline ID of successful snapshots that happened during
+  // a single ongoing page load.
+  int64_t previous_request_id_for_load_;
+
   // If empty, the tab does not have AndroidId and can not capture pages.
   std::string tab_id_;
 
   // The URL of the page that is currently being snapshotted. Used to check,
   // during async operations, that WebContents still contains the same page.
   GURL snapshot_url_;
   // This starts out null and used as a flag for EnsureInitialized() to do the
   // initialization only once.
   std::unique_ptr<SnapshotController> snapshot_controller_;
 
   std::unique_ptr<Delegate> delegate_;
 
   base::WeakPtrFactory<RecentTabHelper> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(RecentTabHelper);
 };
 
 }  // namespace offline_pages
 #endif  // CHROME_BROWSER_ANDROID_OFFLINE_PAGES_RECENT_TAB_HELPER_H_