Offline Page Cache uses user interaction triggers for saving pages.

components/offline_pages/core/snapshot_controller.h

Index: components/offline_pages/core/snapshot_controller.h
diff --git a/components/offline_pages/core/snapshot_controller.h b/components/offline_pages/core/snapshot_controller.h
index 6a7bc64822feebeb3943516716a0917d0f8b4a94..dd0ca01c878dfddcde0dcef4c6c466ac8be3b4e0 100644
--- a/components/offline_pages/core/snapshot_controller.h
+++ b/components/offline_pages/core/snapshot_controller.h
@@ -30,6 +30,21 @@ class SnapshotController {
     SNAPSHOT_PENDING,  // Snapshot is in progress, don't start another.
     STOPPED,           // Terminal state, no snapshots until reset.
   };
+  // The expected quality of a page based on its current loading progress.
+  enum class PageQuality {
+    // Not loaded enough to reach a minimum level of quality. Snapshots taken at
+    // this point are expected to be useless.
+    POOR = 0,
+    // A minimal level of quality has been attained but the page is still
+    // loading so its quality is continuously increasing. One or more snapshots
+    // could be taken at this stage as later ones are expected to have higher
+    // quality.
+    FAIR_AND_IMPROVING,
+    // The page is loaded enough and has attained its peak expected quality.
+    // Snapshots taken at this point are not expected to increase in quality
+    // after the first one.
+    HIGH,
+  };
 
   // Client of the SnapshotController.
   class Client {
@@ -76,8 +91,10 @@ class SnapshotController {
   size_t GetDelayAfterDocumentAvailableForTest();
   size_t GetDelayAfterDocumentOnLoadCompletedForTest();
 
+  PageQuality current_page_quality() const { return current_page_quality_; }
+
  private:
-  void MaybeStartSnapshot();
+  void MaybeStartSnapshot(PageQuality updated_page_quality);
   void MaybeStartSnapshotThenStop();
 
   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
@@ -87,6 +104,10 @@ class SnapshotController {
   size_t delay_after_document_available_ms_;
   size_t delay_after_document_on_load_completed_ms_;
 
+  // The expected quality of a snapshot taken at the moment this value is
+  // queried.
+  PageQuality current_page_quality_ = PageQuality::POOR;
+
   base::WeakPtrFactory<SnapshotController> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SnapshotController);