Add the request picker and a unit test

components/offline_pages/background/BUILD.gn

 # 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.
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
 # GYP: //components/offline_pages/offline_pages.gypi:background_offliner
 static_library("background_offliner") {
   sources = [
     "offliner.h",
     "offliner_factory.h",
     "offliner_policy.h",
     "request_coordinator.cc",
     "request_coordinator.h",
+    "request_picker.cc",
+    "request_picker.h",
     "request_queue.cc",
     "request_queue.h",
     "request_queue_in_memory_store.cc",
     "request_queue_in_memory_store.h",
     "request_queue_store.h",
     "save_page_request.cc",
     "save_page_request.h",
     "scheduler.h",
   ]
 
   deps = [
     "//base",
     "//components/keyed_service/core",
     "//components/offline_pages:offline_pages",
     "//net",
     "//url",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "request_coordinator_unittest.cc",
+    "request_picker_unittest.cc",
     "request_queue_in_memory_store_unittest.cc",
     "request_queue_unittest.cc",
     "save_page_request_unittest.cc",
   ]
 
   deps = [
     ":background_offliner",
     "//base",
     "//base/test:test_support",
     "//components/offline_pages:offline_pages",
     "//testing/gtest",
     "//url",
   ]
 }