Implementing recent pages snapshot capture.

components/offline_pages/snapshot_controller.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 COMPONENTS_OFFLINE_PAGES_SNAPSHOT_CONTROLLER_H_
 #define COMPONENTS_OFFLINE_PAGES_SNAPSHOT_CONTROLLER_H_
 
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/single_thread_task_runner.h"
 
 namespace offline_pages {
 
 // Takes various signals and produces StartSnapshot calls following a specific
 // policy. Can request snapshots multiple times per 'session'. Session can be
 // ended and another one started by calling Reset().
 // Main invariants:
 // - It never starts overlapping snapshots, Client reports when previous
 //   snapshot is done.
 // - The currently worked on (pending) snapshot is always allowed to complete,
 //   the new attempts to start a snapshot are ignored until it does.
 // - Some signals prevent more snapshots to be taken.
 //   OnLoad is currently such signal.
+// - Once Reset() is called on the SnapshotController, the delayed tasks are
+//   reset so no StartSnapshot calls is made 'cross-session'.
 class SnapshotController {
  public:
   // kStateReady - listening to input, will start snapshot when needed.
   // kStateSnapshotPending - snapshot is in progress, don't start another.
   // kStateStopped - terminal state, no snapshots until reset.
   enum class State { kReady, kSnapshotPending, kStopped };
 
   // Client of the SnapshotController.
   class  Client {
    public:
     // Invoked at a good moment to start a snapshot. May be invoked multiple
     // times, but not in overlapping manner - waits until
     // PreviousSnapshotCompleted() before the next StatrSnapshot().
     // Client should overwrite the result of previous snapshot with the new one,
     // it is assumed that later snapshots are better then previous.
-    // Returns true if the snapshot actually started.
-    virtual bool StartSnapshot() = 0;
+    virtual void StartSnapshot() = 0;
 
    protected:
     virtual ~Client() {}
   };
 
   SnapshotController(
       const scoped_refptr<base::SingleThreadTaskRunner>& task_runner,
       SnapshotController::Client* client);
   virtual ~SnapshotController();
 
@@ skipping 26 matching lines @@
   SnapshotController::State state_;
 
   base::WeakPtrFactory<SnapshotController> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SnapshotController);
 };
 
 }  // namespace offline_pages
 
 #endif  // COMPONENTS_OFFLINE_PAGES_SNAPSHOT_CONTROLLER_H_