Create skeleton for the Prefetching store and initial pipeline step.

components/offline_pages/core/client_id.h

Index: components/offline_pages/core/client_id.h
diff --git a/components/offline_pages/core/client_id.h b/components/offline_pages/core/client_id.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_CLIENT_ID_H_
+#define COMPONENTS_OFFLINE_PAGES_CORE_CLIENT_ID_H_
+
+#include <string>
+
+namespace offline_pages {
+
+// Defines a namespace/id pair that allows offline page clients to uniquely
+// identify their own items within adopting internal systems. It is the client's
+// responsibility to keep id values unique within its assigned namespace, but it
+// is not a requirement.
+struct ClientId {
+  ClientId();
+  ClientId(const std::string& name_space, const std::string& id);
+
+  bool operator==(const ClientId& client_id) const;
+
+  bool operator<(const ClientId& client_id) const;
+
+  // The namespace that identifies the client (of course 'namespace' is a
+  // reserved word, so...).
+  std::string name_space;
+
+  // The client specified id that allows it to uniquely identify entries within
+  // its namespace. These values are opaque to offline page systems and not used
+  // internally as an identifier.
+  std::string id;
+};
+
+}  // namespace offline_pages
+
+#endif  // COMPONENTS_OFFLINE_PAGES_CORE_CLIENT_ID_H_