Add support for OfflinePagesDownloadBridge.

components/offline_pages/downloads/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.gypi:offline_pages_ui_adapter
+static_library("offline_pages_ui_adapter") {
+  sources = [
+    "download_ui_adapter.cc",
+    "download_ui_adapter.h",
+    "download_ui_item.cc",
+    "download_ui_item.h",
+  ]
+
+  deps = [
+    "//base",
+    "//components/offline_pages:offline_pages",
+  ]
+}
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "download_ui_adapter_unittest.cc",
+  ]
+
+  deps = [
+    ":offline_pages_ui_adapter",
+    "//base",
+    "//base/test:test_support",
+    "//components/offline_pages:offline_pages",
+    "//testing/gtest",
+  ]
+}