Add base persistence interfaces for Prefetching Offline Pages.

components/offline_pages/core/prefetch/prefetch_constants.h

+// 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_PREFETCH_CONSTANTS_H_
+#define COMPONENTS_OFFLINE_PAGES_CORE_PREFETCH_PREFETCH_CONSTANTS_H_
+
+namespace offline_pages {
+
+// List of states a prefetch item can be at during its progress through the
+// prefetching process. They follow somewhat the order below but some states
+// might be skipped.
+enum class PrefetchItemState {

[fgorski, 2017/05/10 05:02:53]

I don't thing my original problem was addressed. Here is what I meant:
NEW_REQUEST,
SENT_GENERATE_PAGE,
AWAITING_GCM,
RECEIVED_GCM,
SENT_GET_OPERATION,
RECEIVED_BUNDLE,
DOWNLOADING,
FINISHED,
ZOMBIE // or SENTINEL

[carlosk, 2017/05/10 22:48:53]

Done.

I didn't understand your initial concern. Previous names matched the design doc
ones but they are way more legible like this.

+  // New request just received from the client.
+  NEW_REQ,
+  // The item has been included in a GeneratePageBundle RPC requesting the
+  // creation of an archive for its URL.
+  SENT_GPB,
+  // The archive was not immediately available (cached) upon the request and
+  // is now waiting for a GCM message notifying of its archiving operation
+  // completion.
+  WAIT_GCM,
+  // The GCM message notifying of the archiving operation completion was
+  // received for this item.
+  GOT_GCM,
+  // A GetOperation RPC was sent for this item to query for the final results
+  // of its archiving request.
+  SENT_GET_OP,
+  // Information was received about a successfully created archive for this
+  // item that can now be downloaded.
+  RECV_BUNDLE,
+  // This item's archive is currently being downloaded.
+  DL,
+  // Item has finished processing, successfully or otherwise and is waiting to

[fgorski, 2017/05/10 05:02:53]

nit: put a , after otherwise

[carlosk, 2017/05/10 22:48:53]

Done.

+  // be processed for stats reporting to UMA.
+  FIN,
+  // UMA stats have been reported and the item is being kept just long enough
+  // to confirm that the same URL is not being repeatedly requested by its
+  // client.
+  ZOMBIE,
+};
+
+// Error codes used to identify the reason why a prefetch item has finished
+// processing.
+enum class PrefetchItemErrorCode {
+  NO_ERROR,
+};
+
+}  // namespace offline_pages
+
+#endif  // COMPONENTS_OFFLINE_PAGES_CORE_PREFETCH_PREFETCH_ITEM_H_