Add the download component and initial setup

components/download/public/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")
+}
+
+source_set("public") {
+  sources = [
+    "client.h",
+    "clients.h",
+    "download_params.cc",
+    "download_params.h",
+    "download_service.h",
+  ]
+
+  deps = [
+    "//components/keyed_service/core",
+  ]
+
+  public_deps = [
+    "//base",
+    "//net",
+    "//url",
+  ]
+}