Add a Model to the download component.

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",
+    "//components/download/internal/test_support",
   ]
 
   sources = [
     "download_service_impl.cc",
     "download_service_impl.h",
+    "entry.cc",
+    "entry.h",
+    "model.h",
+    "model_impl.cc",
+    "model_impl.h",
+    "noop_store.cc",
+    "noop_store.h",
+    "store.h",
   ]
 
   deps = [
     "//base",
     "//components/download/public",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
 
   visibility = [ "//components/download:unit_tests" ]
 
   sources = [
-    "download_service_impl_unittest.cc",
+    "model_impl_unittest.cc",
   ]
 
   deps = [
     ":internal",
     "//base/test:test_support",
+    "//components/download/internal/test_support",
     "//testing/gtest",
   ]
 }