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

chrome/browser/android/offline_pages/recent_tab_helper.h

Index: chrome/browser/android/offline_pages/recent_tab_helper.h
diff --git a/chrome/browser/android/offline_pages/recent_tab_helper.h b/chrome/browser/android/offline_pages/recent_tab_helper.h
index b672827bec81e42da771daf3c10fa5bbe2a75aee..f17adb003fbde93a98e769d6ad77d48bbd2241ae 100644
--- a/chrome/browser/android/offline_pages/recent_tab_helper.h
+++ b/chrome/browser/android/offline_pages/recent_tab_helper.h
@@ -34,6 +34,8 @@ class RecentTabHelper
   ~RecentTabHelper() override;
 
   // content::WebContentsObserver
+  void DidStartNavigation(
+      content::NavigationHandle* navigation_handle) override;
   void DidFinishNavigation(
       content::NavigationHandle* navigation_handle) override;
   void DocumentAvailableInMainFrame() override;
@@ -55,12 +57,16 @@ class RecentTabHelper
   };
   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,
@@ -73,7 +79,11 @@ class RecentTabHelper
   // Page model is a service, no ownership.
   OfflinePageModel* page_model_;

[dewittj, 2016/08/24 23:01:39]

please document that page_model_ can be null even during normal operation of
this tab helper

[Dmitry Titov, 2016/08/25 00:36:09]

Done.

   // 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_;