Add the download component and initial setup

components/download/internal/BUILD.gn

+# 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.
+
+if (is_android) {
+  import("//build/config/android/config.gni")
+  import("//build/config/android/rules.gni")
+}
+
+static_library("internal") {
+  visibility = [
+    ":*",
+    "//components/download",
+  ]
+
+  sources = [
+    "background_download_service_impl.cc",
+    "background_download_service_impl.h",
+  ]
+
+  deps = [
+    "//base",
+  ]
+}
+
+source_set("unit_tests") {
+  testonly = true
+
+  visibility = [ "//components/download:unit_tests" ]
+
+  sources = [
+    "background_download_service_impl_unittest.cc",
+  ]
+
+  deps = [
+    ":internal",
+    "//base/test:test_support",
+    "//testing/gtest",
+  ]
+}