Add the request picker and a unit test

components/offline_pages/background/request_picker.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_BACKGROUND_REQUEST_PICKER_H_
+#define COMPONENTS_OFFLINE_PAGES_BACKGROUND_REQUEST_PICKER_H_
+
+#include "base/memory/weak_ptr.h"
+#include "components/offline_pages/background/request_coordinator.h"
+#include "components/offline_pages/background/request_queue.h"
+
+namespace offline_pages {
+
+class RequestPicker {

[Dmitry Titov, 2016/05/28 00:36:57]

So the pattern of usage is:

requestPicker->ChooseNextRequest();
... this does async lookup into request queue
... then calls a callback.

Typically, this is just:
requestPicker->ChooseNextRequest(callback)
at which point why not

requestQueue->FetchNextRequest(callback)?

[Pete Williamson, 2016/05/28 00:53:47]

The RequestPicker is just a skeleton so far.  Eventually it will house the logic
for picking which request from the request queue to do next.

You do make a good point that I should pass the callbacks as parameters to the
method being called, (ChooseNextRequest) and not in the constructor, and I will
do that in my next revision to this changelist.

+ public:
+  RequestPicker(RequestQueue* requestQueue,
+                RequestCoordinator::RequestPickedCallback picked_callback,
+                RequestCoordinator::RequestQueueEmptyCallback empty_callback);
+
+  ~RequestPicker();
+
+  // Choose which request we should process next based on the current
+  // conditions, and call back to the RequestCoordinator when we have one.
+  void ChooseNextRequest();
+
+ private:
+  // Callback for the GetRequest results to be delivered.
+  void GetRequestResultCallback(RequestQueue::GetRequestsResult result,
+                                const std::vector<SavePageRequest>& results);
+
+  // unowned pointer to the request queue.
+  RequestQueue* queue_;
+  // Callback for when we are done picking a request to do next

[dougarnett, 2016/05/31 18:03:04]

nit - period at end

+  RequestCoordinator::RequestPickedCallback picked_callback_;
+  // Callback for when there are no more reqeusts to pick.
+  RequestCoordinator::RequestQueueEmptyCallback empty_callback_;
+  // Allows us to pass a weak pointer to callbacks.
+  base::WeakPtrFactory<RequestPicker> weak_ptr_factory_;
+};
+
+}  // namespace offline_pages
+
+#endif  // COMPONENTS_OFFLINE_PAGES_BACKGROUND_REQUEST_PICKER_H_