Add the download component and initial setup

components/download/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")
+}
+
+group("download") {
+  public_deps = [
+    "//components/download/public",
+  ]
+
+  deps = [
+    "//components/download/internal",
+  ]
+}
+
+group("unit_tests") {
+  testonly = true
+
+  deps = [
+    "//components/download/internal:unit_tests",
+  ]
+}