Create skeleton for the Prefetching store and initial pipeline step.

components/offline_pages/core/prefetch/add_unique_urls_task.h

Index: components/offline_pages/core/prefetch/add_unique_urls_task.h
diff --git a/components/offline_pages/core/prefetch/add_unique_urls_task.h b/components/offline_pages/core/prefetch/add_unique_urls_task.h
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+// Copyright 2017 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_CORE_PREFETCH_ADD_UNIQUE_URLS_TASK_H_
+#define COMPONENTS_OFFLINE_PAGES_CORE_PREFETCH_ADD_UNIQUE_URLS_TASK_H_
+
+#include <vector>
+
+#include "base/memory/weak_ptr.h"
+#include "components/offline_pages/core/prefetch/prefetch_store.h"
+#include "components/offline_pages/core/prefetch/prefetch_types.h"
+#include "components/offline_pages/core/task.h"
+#include "url/gurl.h"
+
+namespace offline_pages {
+
+// Task that adds new URL suggestions to the pipeline. URLs are matched against
+// existing ones from any stage of the process so that only new, unique ones are
+// actually added.
+// Fully processed items are kept in the store in the PrefetchItemState::ZOMBIE
+// state until it is confirmed that the client for its namespace is not
+// recommending the same URL anymore to avoid processing it twice. So once the
+// step described above is done, all same namespace items in the ZOMBIE state
+// whose URL didn't match any of the just suggested ones are finally deleted
+// from the store.
+class AddUniqueUrlsTask : public Task {
+ public:
+  AddUniqueUrlsTask(PrefetchStore* store,
+                    const std::vector<PrefetchURL>& prefetch_urls);
+  ~AddUniqueUrlsTask() override;
+
+  void Run() override;
+
+ private:
+  PrefetchStore* prefetch_store_;
+  std::vector<PrefetchURL> prefetch_urls_;
+
+  base::WeakPtrFactory<AddUniqueUrlsTask> weak_ptr_factory_;
+  DISALLOW_COPY_AND_ASSIGN(AddUniqueUrlsTask);
+};
+
+}  // namespace offline_pages
+
+#endif  // COMPONENTS_OFFLINE_PAGES_CORE_PREFETCH_ADD_UNIQUE_URLS_TASK_H_