Modify RecentTabHelper to be always-on and observe the loading of the pages.

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 37 matching lines @@
    public:
     virtual ~Delegate() {}
     virtual std::unique_ptr<OfflinePageArchiver> CreatePageArchiver(
         content::WebContents* web_contents) = 0;
     virtual scoped_refptr<base::SingleThreadTaskRunner> GetTaskRunner() = 0;
     // There is no expectations that tab_id is always present.
     virtual bool GetTabId(content::WebContents* web_contents, int* tab_id) = 0;
   };
   void SetDelegate(std::unique_ptr<RecentTabHelper::Delegate> delegate);
 
+  bool is_page_ready_for_snapshot() const {
+    return is_page_ready_for_snapshot_;
+  }
+
  private:
   explicit RecentTabHelper(content::WebContents* web_contents);
   friend class content::WebContentsUserData<RecentTabHelper>;
 
 
-  void LazyInitialize();
+  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 ReportSnapshotCompleted();
   bool IsSamePage() const;
   ClientId client_id() const;
 
-  // Page model is a service, no ownership.
+  // 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_;
   // 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_