Adding a BUILD file for both gyp and gn for the new background_offlining

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",
+    "request_coordinator.cc",
+    "request_coordinator.h",
+    "scheduler.h",
+  ]
+
+  deps = [
+    "//base",
+    "//components/offline_pages:offline_pages",
+    "//net",
+    "//url",
+  ]
+}