[Offline Pages] Set up the initial prefetching service.

components/offline_pages/core/prefetch/prefetch_service.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_SERVICE_H_
+#define COMPONENTS_OFFLINE_PAGES_CORE_PREFETCH_PREFETCH_SERVICE_H_
+
+#include "components/keyed_service/core/keyed_service.h"
+
+class GURL;
+
+namespace offline_pages {
+
+// Main class for the Offline Pages Prefetching feature, which will ask a server
+// to package content from interesting URLs and send it to Chrome for
+// consumption while offline.
+class PrefetchService : public KeyedService {

[Dmitry Titov, 2017/04/11 21:50:46]

As formulated, the service may only have a single client. In this case, Zine
Suggestions are assumed. If we ever want to use this capability for anything
else (like offlining on Svelte), we can't because there is no 'segregation'
between URLs - for example, RemoveAllUnprocessedURLsToPrefetch()
indiscriminately deletes all URLs.

I wonder if it is easier to add either a 'namespace' (string or enum) or a guid
paired with each url or even a ClientId-like struct, so the client may do
'remove by category' for example?

[dewittj, 2017/04/12 00:22:48]

Done.

+ public:
+  ~PrefetchService() override = default;
+
+  // Called when a consumer has new URLs for the system to prefetch.  Currently
+  // the only consumer is article suggestions.

[Dmitry Titov, 2017/04/11 21:50:46]

can we just remove use of 'suggestions' and just use 'pages' or 'urls'?

[dewittj, 2017/04/12 00:22:48]

Done.

+  virtual void OnNewURLsToPrefetch(std::vector<GURL> suggested_urls) = 0;

[jianli, 2017/04/11 22:39:55]

nit: Please add const &.

[dewittj, 2017/04/12 00:22:48]

Done.

+
+  // Called when all existing suggestions are no longer considered valid.  The
+  // prefetch service should remove any suggestions that have not yet started
+  // downloading.
+  virtual void RemoveAllUnprocessedURLsToPrefetch() = 0;

[jianli, 2017/04/11 22:39:55]

Since class name is PrefetchService, we can probably drop "ToPrefetch" from
method name.

[dewittj, 2017/04/12 00:22:49]

I updated it to refer to the PrefetchURL struct instead. WDYT

+};
+
+}  // namespace offline_pages
+
+#endif  // COMPONENTS_OFFLINE_PAGES_CORE_PREFETCH_PREFETCH_SERVICE_IMPL_H_